Ingredients
Tart Base
- 1 sheet puff pastry thawed but cold, about 14 oz
- 1 large egg beaten with 1 tbsp water for egg wash
- Caramelized Onion Layer
- 2 tbsp olive oil
- 1 tbsp unsalted butter
- 1 large yellow onion very thinly sliced
- 1/2 tsp kosher salt
- 1 tsp fresh thyme leaves
Mushroom Topping
- 12 oz chestnut mushrooms from KC Mushroom Co., thinly sliced
- 2 tbsp olive oil
- 1 tbsp unsalted butter
- 2 cloves garlic minced
- 2 tsp fresh thyme leaves
- 1 tsp balsamic vinegar
- kosher salt and black pepper to taste
Cheese and Finish
- 4 oz Gruyère freshly grated
- 2 oz Parmigiano-Reggiano freshly grated
- extra fresh thyme leaves to garnish
- flaky sea salt
Instructions
- Preheat the oven to 400°F. Line a rimmed baking sheet with parchment.
- For the caramelized onions: heat the olive oil and butter in a skillet over medium-low. Add the sliced onions and salt and cook slowly, stirring every few minutes, for 20 to 25 minutes — until deeply golden, jammy and reduced to about a quarter of their original volume. Add the thyme leaves in the last minute. Set aside to cool slightly.
- While the onions caramelize, work on the mushrooms. Heat the olive oil and butter in a separate large skillet over medium-high. Add the sliced mushrooms in a single layer and leave them undisturbed for 3 to 4 minutes. Toss and cook another 3 minutes until deeply golden and the pan is dry.
- Add the garlic, thyme and balsamic vinegar. Stir for 30 seconds until fragrant. Season with salt and pepper. Set aside to cool slightly — wet, hot toppings on cold pastry will give you a soggy tart.
- Roll the puff pastry out on the parchment to a rough rectangle about 10 by 14 inches. Use a knife to score a half-inch border around the edge — don't cut all the way through. Prick the interior all over with a fork to prevent it from puffing up too much in the center.
- Brush the scored border with the egg wash.
- Sprinkle half the Gruyère evenly over the interior of the tart, staying within the scored border. Spread the caramelized onions over the cheese in an even layer.
- Pile the seared mushrooms over the onions, then scatter the remaining Gruyère and the Parmesan over everything.
- Bake on the middle rack for 22 to 25 minutes, until the pastry border is deeply golden and puffed and the cheese is bubbling and just starting to brown at the edges.
- Let rest for 5 minutes. Garnish with fresh thyme and a sprinkle of flaky salt. Slice into squares or rectangles and serve warm.
Notes
From the farm: Chestnut mushrooms work beautifully in a tart because they retain their texture during baking rather than turning to mush. The pre-sear is essential — raw mushrooms on top of pastry release water as they bake and turn everything soggy.
Make ahead: The caramelized onions and seared mushrooms can both be prepared up to 24 hours in advance and refrigerated separately. Assemble and bake the tart fresh for best texture.
Tip: A truly cold puff pastry is the secret to maximum puff. If your kitchen is warm, return the assembled tart to the fridge for 10 minutes before baking.
Variation: Top the finished tart with a handful of arugula dressed in lemon and olive oil for a peppery, fresh contrast. Or finish with a drizzle of truffle oil for a special-occasion version.
