Name: Upside-Down Onion and Gruyere Tarts
Prep time: 20 min
Cook time: 35 min
Total time: 55 min
Yield: 4 servings
Cuisine: French
Ingredients: 2 large yellow onions, thinly sliced; 2 tablespoons unsalted butter; 1 tablespoon olive oil; 1 teaspoon fresh thyme leaves; 1 sheet puff pastry, thawed; 1 cup grated Gruyere cheese; 1 teaspoon balsamic vinegar; 1/4 teaspoon salt; 1/4 teaspoon black pepper; Flour, for dusting

Let Me Tell You About These Upside-Down Onion and Gruyere Tarts

Okay, so you know how some recipes just stick with you after you try them once—like the way the smell of caramelized onions clings to your shirt for what feels like forever? That’s these tarts for me. The first time I made them, I managed to burn the onions a bit (had wandered off to chase my dog out of the pantry, ha), but you know what? They still tasted incredible. My mum once said they reminded her of a French bakery we stumbled into on a rainy trip to Lyon… though honestly, I think she was just happy not to be the one doing dishes. Anyway, if you’ve never tried making upside-down tarts, this one’s a cracker, and perfect for those evenings when you want to impress but not break your brain.

Why You’ll Love This Tart (or at least why I do!)

I make this on those chilly evenings when I want something cozy but can’t face another soup. It’s perfect for when I fancy a bit of French flair without spending all day faffing about. My family goes mad for it—especially since it tastes like it took hours but actually, it’s a bit of a cheat’s version. Oh, and if you’re on the fence about onions (my cousin claims she can’t stand them, yet she somehow hoovers up half the tart every time), the Gruyere sort of melts everything together. Also, once or twice (okay, maybe more), I’ve forgotten to chill the pastry first, and it still mostly works out—just slightly more rustic.

What You’ll Need (But Honestly, Improv is Fine)

4-5 medium yellow onions (Red works too, I just use what’s lurking in my veg bin)
2 generous tablespoons of butter (or olive oil, if you’d rather, though butter’s my usual—it’s a flavor thing)
1 or 2 tsp brown sugar (I sometimes skip this if the onions are sweet enough)
Salt and a grind of fresh pepper
Fresh thyme (dried’s fine in a pinch, but fresh is the bee’s knees)
About 100g Gruyere cheese, grated (Or Comté; heck, Cheddar worked when that was all I had)
1 sheet of ready-rolled puff pastry (homemade if you’re feeling industrious, but honestly, store-bought is grand; my gran swears by Jus-Rol but I say use what you can get)
A splash of balsamic vinegar (optional, but adds a lovely tang)
1 egg, beaten, for brushing (but if you forget, nothing explodes—just less golden)

How To Make Them (And How to Fix Things If You Get Distracted)

Preheat your oven to about 200°C (fan 180°C, or the whatever mark on your oven that actually works; mine’s got moods). Find a sturdy non-stick frying pan, or just use an oven-safe baking tin.
Slice the onions into thick-ish rounds—don’t fuss if some are wonky, it’s rustic food (and my knife skills aren’t exactly chef-quality anyway!).
Pop the butter in your pan on a medium low heat. Once it starts to foam, chuck in your onions. Let them cook gently, spreading out so most touch the pan. This is where I sneak a taste even though I know it’s too soon.
Add the brown sugar, a good pinch of salt, and lots of black pepper. At this point, resist the urge to poke too much: let the onions get golden and gorgeous, which takes about 15–20 mins. If they catch a bit on the edges, you can call it extra flavor rather than a mistake (which is what I do every time). If you’re using thyme, sprinkle in the leaves about halfway.
When the onions are meltingly soft and golden, take off the heat and splash in some balsamic if you’re feeling fancy. Give it a little wiggle to distribute everything nicely.
Scatter over most of the cheese (save a handful for the top) and then roll your pastry over the onions. Tuck in any scraggly corners; if it looks a bit odd, trust me, it all comes together. Actually, I find it better if the pastry covers the onions right up to the pan edge—less leakage.
Brush with egg wash (that is, if you remembered) and poke a couple of small holes for steam—don’t skip this unless you enjoy soggy pastry.
Bake in the hot oven for about 25–30 minutes or until the top’s all puffed and golden and you can’t resist sneaking a look through the oven window.
Now for the fun: carefully flip the tart onto a plate (put a plate over the pan and flip, holding your breath). Some onions might try to stick—just tuck ’em back in. That’s character! Top with the reserved cheese and maybe more thyme if you fancy.

Notes Only a Tired Cook Would Know

If you use a tin with a loose base, line it, or risk the Great Butter Flood of 2020 (ugh—what a cleanup).
I sometimes forget the thyme; it’s still great, just less “herbaceous,” as posh folk say.
Cleaning the pan is easier if you do it right away, but let’s be real, it usually soaks overnight in my sink.

Some Variations (And What Worked… or Didn’t)

I’ve swapped in red onions and even shallots; both work and look pretty swish.
Tried feta once—too salty for me, but my mate liked it. Go figure.
Added mushrooms once (thought I was brilliant). Bit too soggy, so maybe don’t try that one unless you dry-roast them first.

Equipment (Plus the Best Workarounds)

If you’ve got a non-stick frying pan that’s ovenproof, you’re golden. If not, use any old cake tin or pie dish—just grease it really well (or use baking parchment, which I sometimes have to rescue from beneath a pile of mismatched Tupperware). Don’t have a rolling pin? Wine bottles do in a pinch. Ask me how I know!

How to Store It? (It Rarely Lasts That Long)

Honestly, in my house this tart vanishes almost straight out of the oven. But if you do have leftovers, wrap them up and pop them in the fridge—they warm up nicely (180°C for 10ish mins, or microwave if you must). I think the flavors kind of bloom overnight, but usually, someone nicks the last piece before breakfast.

How We Like to Serve It (Yours Might Be Different)

I like mine with a green salad and maybe a few pickles on the side—cuts through the cheesiness and makes it feel like a “proper” meal. Sometimes, I’ll make a quick mustardy vinaigrette. On lazy days? Just with a cup of builder’s tea and a napkin tucked under my chin.

Lessons Learned (aka Things I Messed Up So You Don’t Have To)

I once tried to shortcut the onion-cooking (was late for meeting friends at the pub). Tart tasted harsh; take the time, it’s worth it.
Don’t skip the egg wash if you want that pro pastry look, but it’s not *essential*.
Flipping too soon after baking? Disaster. Let it rest a bit so the onions behave.

FAQ (Yes, People Actually Ask These!)

Can I use other cheese?: Yep, totally. I’ve tried Cheddar, Swiss, Comté—just avoid anything that melts into oil puddles. Blue cheese works but can be punchy.
Is it okay to make ahead?: For sure! Make it a day ahead, then warm through in a low oven. Actually, I think this tastes better the next day.
What goes with these tarts?: Light salad, or even a simple soup, but anything’s fair game (even just crisps if that’s your thing).
My tart stuck—help!: It happens (happens to me too; more than I’d admit). Just gently loosen edges with a knife—any stubborn bits are still tasty, promise.
Any more resources?: Check out Serious Eats’ onion tart tips or Smitten Kitchen’s onion & Gruyere ideas if you fancy noodling further.

Oh—and before I forget, did you know you can grow mini onions in a mug on your windowsill? Not exactly relevant, but it’s a fun project if you, like me, accidentally buy too many and hate waste.

Upside-Down Onion and Gruyere Tarts