Preheat the oven to 375 F and place the rack in the lower third for even baking.

Fit the pie crust into a 9-inch pie dish or tart pan, crimping edges, and prick the base with a fork.

Line the crust with parchment and fill with pie weights or dried beans. Blind bake for 12 to 15 minutes until the edges look set. Remove weights and parchment, then bake for 5 more minutes. Let cool slightly.

While the crust bakes, warm oil or butter in a skillet over medium heat. Cook onion with a pinch of salt until soft and lightly golden, about 6 to 8 minutes.

Stir in garlic for 30 seconds, then add the spinach and cook until wilted and most moisture evaporates, about 2 to 3 minutes. Transfer to a plate and let cool, then squeeze out extra liquid if needed.

In a bowl, whisk ricotta until smooth. Add eggs, cream, and milk, whisking until evenly combined.

Stir in Gruyere, Parmesan, nutmeg, salt, pepper, and optional lemon zest or red pepper flakes.

Scatter the spinach and onion mixture over the par-baked crust. Pour the ricotta custard over the top and gently smooth.

Bake for 35 to 45 minutes, until the center looks set but still has a soft wobble. If you check with a thermometer, the center should be near 165 F.

Let the quiche rest for 15 minutes before slicing to allow the custard to finish setting.