In a mixing bowl, combine the all-purpose flour and salt. Using a pastry cutter or your fingers, cut in the cold, cubed butter until the mixture resembles coarse crumbs.

Gradually add ice water, one tablespoon at a time, mixing just until a dough forms. Shape the dough into a disk, wrap in plastic, and chill for 30 minutes.

Roll out the chilled dough on a lightly floured surface and fit it into a 9-inch tart or pie pan, trimming the edges as needed.

Use a fork to prick the base, preventing it from puffing up during baking. Line the crust with parchment paper and fill it with baking weights or dried beans.

Bake in a preheated oven at 375°F (190°C) for 15 minutes. Remove the weights and parchment, then bake for an additional 5 minutes until lightly golden.

In a skillet, heat the olive oil over medium heat. Add the finely chopped onion and sauté until softened, about 3–4 minutes.

Stir in the chopped spinach and cook until it wilts, about 2 minutes. Remove from heat and let it cool slightly.

In a large mixing bowl, whisk together the eggs, heavy cream, milk, salt, black pepper, and optional nutmeg.

Gently stir in the sautéed spinach and onion mixture along with the shredded cheese.

Pour the filling into the pre-baked crust, smoothing the top for an even finish.

Place it in the oven and bake at 350°F (175°C) for 35–40 minutes, until the quiche is set in the center and golden on top.

Once baked, remove the quiche from the oven and allow it to cool for at least 10–15 minutes.

Serve warm or at room temperature.