Follow the package directions for defrosting the puff pastry. Do not unfold pastry until fully thawed to avoid cracking.

For the mushrooms, heat a 12-inch Stainless Clad Frying Pan over medium-high. Add olive oil and, once glistening, add in mushrooms. Working in batches to avoid crowding the pan, sauté your mushrooms. Season mushrooms with salt to help release any extra moisture.

When your mushrooms are cooked ⅔ of the way through, add 1 Tbsp. butter to the pan, along with the yellow onion, shallots, and ⅔ chopped thyme. Cook for 5 minutes, then add garlic and half the lemon zest. Add the lemon juice to deglaze the pan. Taste and season with salt and pepper.

Remove the mixture from heat and transfer to a mixing bowl to let cool. Make sure the filling is at room temperature before assembling the tart so it doesn’t melt the puff pastry.

Preheat oven to 400F. Lightly butter the Baking Slab and place the puff pastry inside (use a Rolling Pin to roll out the pastry to fit, if necessary.) You will want ½-inch overhang for the crust. Using a fork, dock the surface of your dough by poking small holes to help release steam and avoid puffing while baking. Fold excess dough over itself to form a thicker crust around the rim.

Add 4 oz. each Gruyère and Parmesan to the cooled mushrooms and stir to combine. Sprinkle 2 oz. each Gruyère and Parmesan over the base of the tart.

Spoon the mushroom mixture into the tart, then top with the remaining cheese.

Bake until golden brown, 30–40 minutes. Once the tart has slightly cooled, garnish with remaining chopped thyme and lemon zest.