01 - Set oven temperature to 375°F (190°C).
02 - Roll out the pastry sheet and fit into a 9-inch tart or pie pan. Trim excess dough and prick base with a fork.
03 - Line pastry with parchment paper and fill with baking weights or dried beans. Bake for 10 minutes. Remove weights and paper, then bake an additional 5 minutes.
04 - Whisk together eggs, heavy cream, whole milk, salt, black pepper, and nutmeg until smooth.
05 - Evenly distribute grated cheese, chopped spinach, sautéed onions or leeks, and diced bacon or ham (if used) over the baked pastry shell.
06 - Pour the egg and cream mixture evenly over the fillings in the crust.
07 - Bake for 30 to 35 minutes until the filling is set and the top is golden brown.
08 - Let the tart rest for 10 minutes before slicing and serving.