Savory Mushroom Onion Tart

There's something about the smell of onions turning golden that makes a kitchen feel like home. Years ago, I watched my neighbor pull a tart from her oven—the pastry was impossibly flaky, the filling studded with caramelized onions that had lost their sharpness to time and heat. She sliced it warm and handed me a wedge with a knowing smile, and I understood then that the simplest dishes often carry the most pleasure. This tart became my answer to that moment.

I made this for a dinner party once when I was short on time and low on confidence, but I had puff pastry and mushrooms. My guests lingered at the table longer than expected, asking for the recipe, and I realized that sometimes the most memorable meals aren't the ones that demand hours in the kitchen—they're the ones that come together with honesty and care.

Ingredients

• Ready-made puff pastry (1 sheet, about 250 g): Thawed pastry is your shortcut to golden, flaky layers without the fuss; don't skip the chilling step, as it keeps the butter in the dough separated and puffed.
• Olive oil (2 tablespoons) and unsalted butter (1 tablespoon): Together they build a rich, nutty flavor base for the vegetables without the pastry becoming greasy.
• Yellow onions (2 large, thinly sliced): Slice them thin so they cook evenly and collapse into sweet strands; this is where patience pays off.
• Cremini or button mushrooms (400 g): Their earthiness balances the sweetness of the onions, and slicing them thin ensures they'll release moisture and caramelize rather than steam.
• Garlic (2 cloves, minced): Added at the end so it stays fragrant and doesn't burn, playing a supporting role to the onions and mushrooms.
• Eggs (3 large), heavy cream (150 ml), and Gruyère cheese (100 g): This custard base sets gently around the vegetables, creating a creamy filling that's rich without being heavy.
• Fresh thyme leaves (1 tablespoon or 1 teaspoon dried): The herb that whispers rather than shouts, tying everything together with an earthy thread.
• Salt (1 teaspoon) and black pepper (1/2 teaspoon): Season generously at each step so flavors build rather than appear only at the end.
• Fresh chives or parsley (optional): A final shower of green adds brightness and a hint of freshness that cuts through the richness.

Instructions

Prep your pastry shell:

Preheat your oven to 200°C (400°F) and line a 23 cm tart pan with parchment. Roll out the thawed puff pastry, press it gently into the pan, trim the edges, and prick the base all over with a fork—this lets steam escape so the bottom doesn't puff up unevenly. Slide it into the fridge while you work on the filling; cold pastry puffs higher and more evenly.

Caramelize the onions slowly:

Heat olive oil and butter in a large skillet over medium heat until the butter foams. Add your sliced onions and let them cook, stirring often, for about 12 minutes until they're soft, golden, and sweet—this is the magic step, so don't rush it or crank up the heat.

Build the vegetable layer:

Add the sliced mushrooms to the onions and cook for 8–10 minutes, stirring occasionally, until they release their water and turn golden at the edges. Stir in the minced garlic and thyme for the last minute, then set everything aside to cool just slightly so you don't scramble the eggs later.

Make the silky custard:

In a bowl, whisk together the eggs, cream, salt, and pepper until smooth. Stir in the grated Gruyère—the warmth of the vegetables will help it melt throughout the filling, creating pockets of cheesy richness.

Assemble and bake:

Spread the cooled onion-mushroom mixture evenly over your chilled pastry shell, then pour the custard mixture over the top. Bake for 25–30 minutes until the filling is just set (it should jiggle slightly in the very center) and the pastry is deep golden brown.

Rest and serve:

Let the tart cool for 10 minutes before you slice it—this gives the filling time to set completely. Finish with a scatter of fresh herbs if you like, then serve warm or even at room temperature.

Save

There was an evening when I pulled this tart from the oven and the whole kitchen fell silent for a moment—just the sound of it cooling, the pastry crackling softly as it settled. My daughter asked if she could have another slice before anyone else, and I knew I'd found something worth making again and again.

Why This Tart Works

The genius of this dish is that it relies on technique, not a long ingredient list. The onions and mushrooms do most of the talking—once they're caramelized and golden, the custard and cheese simply support what's already delicious. You're building layers of flavor that taste like they took hours, but actually reward you for a little patience and steady heat.

Make It Your Own

This tart is forgiving and flexible, which is why it works as both a weeknight dinner and something you'd serve to guests. If you love wine, add a splash of dry white wine when the mushrooms are cooking—it deepens their flavor and adds a subtle sweetness. Swap the Gruyère for sharp cheddar, creamy goat cheese, or even a crumble of feta if you want to shift the tone of the whole dish.

Serving and Storage

Serve this warm, at room temperature, or even the next day—it's one of those dishes that tastes almost better after it's had time to settle. Pair it with a simple green salad dressed with sharp vinaigrette to cut through the richness, and a chilled glass of white wine if you're feeling it.

• Leftovers keep for three days in an airtight container in the fridge and reheat gently in a 160°C oven until just warm.
• If you're making this for a crowd, you can assemble it hours ahead and bake it just before serving—the tart pan stays happy in a cool oven waiting.
• Don't stress about perfect slices; rustic is more forgiving and often more beautiful anyway.

Save

This tart reminds me that the best meals come from honoring simple ingredients and giving them time to become something more. It's the kind of dish that brings people to the table not because it's complicated, but because it's real.

Recipe FAQ

→ What type of pastry works best for this tart?

Ready-made puff pastry is ideal for a flaky, buttery crust that complements the rich filling perfectly.

→ Can I substitute the cheese?

Gruyère can be replaced with Swiss, goat cheese, or feta for a different flavor profile while maintaining creaminess.

→ How should the mushrooms be cooked for best flavor?

Sauté mushrooms until they release moisture and turn golden to enhance their earthy taste and texture.

→ Is there a way to enhance the onion flavor?

Slowly caramelizing the onions over medium heat brings out their natural sweetness and depth.

→ What herbs complement this tart?

Fresh thyme is used in the filling, while chopped chives or parsley add a fresh garnish to brighten flavors.

→ Can this tart be served at room temperature?

Yes, it is delicious both warm or at room temperature, making it flexible for various serving occasions.