Whisk flour and 1/4 tsp salt in a medium bowl. Add cold cubed butter and cut in with a pastry cutter or two forks until mixture resembles coarse meal with some pea-sized butter pieces remaining.

Add water and chill

Sprinkle 3 tablespoons ice water over the flour-butter mixture, stirring gently until dough just comes together when pressed. Do not overwork. Gather into a disc, wrap in plastic, and chill in the refrigerator for at least 30 minutes.

Roll and line pan

Preheat oven to 375°F (190°C). On a lightly floured surface, roll the chilled dough to about 1/8-inch thickness. Fit into a 9-inch (23 cm) round tart pan or a small rectangular pan if making slices; trim excess and crimp edge. Refrigerate 10 minutes.

Blind bake

Line the crust with parchment and fill with pie weights or dried beans. Bake for 12 minutes, remove weights and parchment, then bake another 6–8 minutes until lightly golden. Let cool slightly.

Filling

Prepare fillings

If using bacon or ham, chop and cook until lightly crisp. Sauté chopped onion or shallot in 1 tbs olive oil or melted butter over medium heat until translucent, 3–4 minutes; cool. Wilt spinach in the same pan briefly, squeeze out excess moisture and chop. Grate the Gruyère.

Mix custard

In a bowl, whisk together 3 eggs, 3/4 cup whole milk, and 1/4 cup heavy cream until smooth. Stir in 1/2 tsp salt, 1/4 tsp black pepper and 1/8 tsp freshly grated nutmeg.

Assemble quiche

Evenly spread the cooked bacon or ham (if using), sautéed onion, and wilted spinach in the pre-baked crust. Sprinkle the grated Gruyère over the fillings. Pour the egg-and-cream custard gently over the cheese and fillings, leaving about 1/8 inch from the rim so it doesn't spill.

Final bake

Brush the crust edge with egg wash. Bake at 375°F (190°C) for 25–30 minutes, or until the custard is set but still slightly wobbly in the center and the top is lightly golden. If the crust edge browns too quickly, cover with foil.

Rest

Remove from oven and let the quiche rest on a rack for 15 minutes to finish setting. This makes it easier to slice into clean quiche slices.

Finishing

Slice and serve

Run a thin knife around the edge to loosen, then cut into 6 equal slices. Serve warm or at room temperature. Leftovers keep covered in the refrigerator for up to 3 days; reheat gently in a 325°F (160°C) oven for 10–12 minutes.