In a medium bowl, combine 1.5 cups all-purpose flour and 1 tsp salt. Cut 6 tbsp cold unsalted butter (cut into small cubes) into the flour with a pastry cutter or fingers until mixture resembles coarse crumbs with some pea-sized pieces remaining.

Sprinkle 2 tbsp ice water over the flour-butter mixture, toss gently with a fork until dough just comes together. If needed, add up to 1 tbsp more water, 1 tsp at a time. Do not overwork.

Form dough into a disk, wrap in plastic, and chill in refrigerator for at least 30 minutes.

Preheat oven to 375°F (190°C). On a lightly floured surface, roll chilled dough to a 12-inch circle about 1/8-inch thick. Fit into a 9-inch (23 cm) tart pan or pie plate with removable bottom, trim and crimp edges. Chill crust in freezer 10 minutes.

Line crust with parchment and fill with pie weights or dried beans. Blind-bake for 15 minutes, remove weights and parchment, then bake 5–7 more minutes until lightly golden. Let cool slightly.

Filling

While crust chills, heat 1 tbsp unsalted butter in a small skillet over medium heat. Add 1/2 cup finely chopped onion and cook, stirring occasionally, until softened and translucent, about 5–7 minutes. Remove from heat and cool slightly.

Spread 1.5 cups diced smoked ham evenly over the partially baked crust. Sprinkle 1.5 cups grated Gruyère evenly over the ham. Scatter the sautéed onions on top.

Custard

In a medium bowl, whisk together 4 eggs until combined. Whisk in 1 cup whole milk and 1/2 cup heavy cream until smooth.

Season the custard with 1/2 tsp salt, 1/4 tsp freshly ground black pepper, and 1/8 tsp freshly grated nutmeg. Taste and adjust seasoning lightly.

Pour the custard slowly over the ham, cheese, and onions in the crust. Leave a small gap near the rim to prevent overflow. Fill until just below the rim (about 3/4 full).

Let the quiche rest 5 minutes to allow any large air bubbles to dissipate; gently skim bubbles with a spoon.

Finishing

Whisk 1 egg with 1 tsp water to make an egg wash. Brush the crust edge lightly with egg wash for a golden finish.

Bake quiche in preheated 375°F (190°C) oven until custard is set but still slightly wobbly in the center, about 30–35 minutes. If crust edge browns too quickly, cover edges with foil.

Remove quiche from oven and cool on a rack for at least 15 minutes before slicing. This helps the custard finish setting.

Serve warm or at room temperature. Leftovers refrigerate up to 3 days; reheat gently in a 325°F (160°C) oven until warmed through.