In a large skillet, heat the olive oil over medium heat. Add the sliced leeks and diced onion. Sauté for about 5-7 minutes until softened. Season with salt, pepper, and thyme. Remove from heat and let cool slightly.
In a mixing bowl, whisk together the heavy cream and eggs until well combined. Stir in the shredded Gruyère cheese.
Add the sautéed leeks and onions to the cream and egg mixture, stirring until evenly distributed.
Place the pie crust in a 9-inch tart pan or pie dish. Pour the leek and cheese mixture into the crust, spreading it evenly.
Sprinkle the grated Parmesan cheese on top.
Bake in the preheated oven for 30-35 minutes, or until the tart is set and the top is golden brown.
Allow the tart to cool for about 10 minutes before slicing and serving.
Notes
For a lighter version, substitute half-and-half for the heavy cream.
You can also add cooked bacon or ham for extra flavor.
If you prefer a gluten-free option, use a gluten-free pie crust.