Preheat your oven to 375°F (190°C).

Fit the chilled pie crust into a 9-inch pie dish. Crimp the edges and trim any excess.

To keep the crust from puffing, line it with parchment and fill with pie weights or dried beans.

Blind-bake for 12–15 minutes, until the edges are set and the bottom is just starting to color. Remove weights and bake 3–4 more minutes if the center looks wet. Let cool slightly while you make the filling.

Heat 1 tablespoon olive oil in a large skillet over medium heat.

Add the chopped onion and a pinch of salt. Sauté until soft and translucent, about 5 minutes.

Stir in the garlic and cook 30 seconds more.

Add the spinach in batches, stirring until wilted. If using frozen, add the thawed, squeezed spinach and cook until warmed through.

Transfer cooked spinach to a bowl and squeeze out any excess moisture using a clean kitchen towel or paper towels. Chop roughly if you like smaller pieces.

In a bowl, whisk together the ricotta, eggs, milk (or half-and-half), grated Parmesan, salt, pepper, and nutmeg.

Fold in the cooled spinach mixture and lemon zest if using. Taste and adjust seasoning.

Pour the filling into the pre-baked crust. Sprinkle the mozzarella on top if you want a golden, melty finish.

Bake at 375°F for 35–40 minutes, or until the center is just set (it should jiggle slightly).

Let the quiche rest for 10–15 minutes before slicing.

Slice and serve warm with a crisp green salad, roasted tomatoes, or a light soup.