Directions

• Preheat the oven to 400°F. To a pan, add about a tablespoon of olive oil. Warm over medium heat and once hot, add in the onion slices. Cook until the onions begin to become golden along the edges and caramelize.
• Add the mushroom slices to the pan and cook for another few minutes until the mushrooms are tender. Turn off the heat and temporarily set the pan aside.
• Unroll the pie crust onto a baking sheet fitted with parchment paper or a silicone baking mat. Brush the egg white over most of the pie crust, leaving about a 1 1/2 inch perimeter bare. Let the crust rest for a minute for the egg white to slightly dry.
• Spread the ricotta cheese all over the area covered with the egg white. Use a zester to finely grate a little bit of parmesan cheese as a slight garnish over all the ricotta cheese. Add a small pinch of salt and freshly ground pepper, as well as the pinch of nutmeg, over the cheese.
• Distribute the mushroom-onion mixture all over the ricotta cheese, then top that with the shredded gruyere. Add the thyme leaves all over the top.
• Fold the edges of the pie crust over the mushroom filling, all along the edges. Brush the egg yolk over the pastry, then top the pastry with some of the finely grated parmesan.
• Bake the galette for 20-30 minutes. If you're using store-bought crust, it will typically look golden and be ready in the 20-25 minute mark, whereas homemade pastry can take up to 30 minutes.
• Let the galette rest for 10 minutes on the baking sheet, then transfer to your serving plate.