In a medium bowl combine flour, salt, and sugar. Add cold butter cubes and use a pastry cutter or fingers to cut butter into flour until mixture resembles coarse crumbs with some pea-sized pieces remaining.

Add water and chill

Sprinkle cold water over the mixture a tablespoon at a time, tossing gently until dough just comes together when pressed (do not overwork). Form into a disk, wrap in plastic, and refrigerate for at least 30 minutes (up to 2 hours).

Blind-bake crust

Preheat oven to 375°F (190°C). On a lightly floured surface roll chilled dough to a 12-inch circle and fit into a 9-inch (23 cm) tart or pie pan with 1-inch sides. Trim and crimp edges. Chill crust in pan for 10 minutes while oven finishes preheating. Line crust with parchment paper or foil and fill with pie weights or dried beans. Bake 15 minutes, remove weights and parchment, then bake an additional 6–8 minutes until the crust is just set and lightly golden. Brush inside edges with milk or egg wash if desired and return to oven 1–2 minutes to set. Remove and cool slightly.

Filling

Prepare spinach

Heat 1 tablespoon olive oil in a large skillet over medium heat. Add minced garlic and sauté 30 seconds until fragrant. Add the packed spinach in batches and cook, stirring, until wilted and bright green (about 3–4 minutes total). Transfer spinach to a colander and press or squeeze with a spoon to remove excess liquid. When cool enough to handle, chop coarsely and transfer to a bowl.

Combine vegetables and cheeses

To the chopped spinach add diced roasted red bell pepper, halved Kalamata olives, crumbled feta, and grated Gruyère. Stir to combine evenly.

Make custard

In a separate bowl whisk together eggs, whole milk, heavy cream, dried oregano, nutmeg, kosher salt, and black pepper until smooth and slightly frothy.

Assemble quiche

Scatter the spinach-cheese-olive-pepper mixture evenly in the pre-baked crust. Pour the egg-milk custard slowly over the filling so it settles into gaps. Gently tap the pan on the counter to remove air bubbles and level the surface. Sprinkle a little extra grated Gruyère on top if desired.

Bake

Reduce oven temperature to 350°F (175°C). Bake quiche on the middle rack for 30–38 minutes, or until the center is set but still slightly wobbly (an instant-read thermometer inserted in center should read about 165°F/74°C). If the crust edge browns too quickly, tent with foil.

Rest

Remove quiche from oven and cool on a wire rack for 10–15 minutes to finish setting. This improves slicing.

Finishing

Garnish the quiche with chopped fresh parsley if using. Slice into 6–8 wedges and serve warm or at room temperature.