In a large mixing bowl, whisk together the 1 ¼ cups all-purpose flour and ½ tsp salt. Add the cold ½ cup unsalted butter cold cubes and use your fingertips or a pastry cutter to work the ½ cup unsalted butter cold into the 1 ¼ cups all-purpose flour until the mixture resembles coarse, pea-sized crumbles — some larger chunks are fine. This creates flakiness.

Drizzle in the 3 tbsp ice water one tablespoon at a time, stirring gently with a fork after each addition. Stop when the dough just comes together and holds when pressed — it should not be sticky or wet. Form into a flat disc, wrap in plastic wrap, and refrigerate for at least 30 minutes (or up to 2 days).

Preheat the oven to 375°F. On a lightly 1 ¼ cups all-purpose flour-dusted surface, roll the chilled dough into a circle about 12 inches in diameter and roughly 1/8-inch thick. Carefully transfer to a pie dish, pressing gently into the bottom and sides. Trim any overhang to about 1/2 inch and fold it under itself to create a neat edge. Crimp decoratively with your fingers or a fork.

Blind bake the crust: Line the inside of the crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes, then carefully remove the parchment paper and weights. Bake for another 5–8 minutes, until the bottom looks dry and just barely golden. Remove from the oven. If using, brush the bottom of the warm crust with a thin layer of Dijon mustard — this adds flavor and creates a moisture barrier.

Line a baking sheet with aluminum foil, extending it up the sides to catch grease. Optionally place a wire rack on top for better airflow and crispier bacon. Lay strips in a single layer — close but not overlapping. No oil or extra seasoning needed.

Set oven to 400°F. You can start bacon in a cold oven as it preheats, or wait until fully preheated — both work. Use an external meat thermometer if your oven runs inaccurate. Start checking at 14–15 minutes. Thin-cut: 10–14 min. Standard-cut: 15–20 min. Thick-cut: 20–22 min. No need to flip. Check every couple of minutes after the first check — overdone happens fast. Pull bacon just before it looks done; it keeps cooking for a minute or two off the heat.

In the same skillet with the reserved 6 oz thick-cut bacon cut into lardons fat, sauté the diced small yellow onion finely diced over medium-low heat until softened and translucent, about 5 minutes. Do not let it brown. Remove from heat and let cool slightly.

In a large mixing bowl or large measuring cups (for easy pouring), whisk together the 4 eggs, 1 ¼ cups heavy cream, and ¼ cup whole milk until smooth and fully combined. Season with ½ tsp salt, ¼ tsp black pepper freshly ground, and freshly grated nutmeg freshly grated. Whisk again to incorporate.

Scatter the cooked 6 oz thick-cut bacon cut into lardons and sautéed 1 small yellow onion finely diced evenly over the bottom of the pre-baked crust. Sprinkle the grated Gruyère cheese freshly grated evenly over the top of the thick-cut bacon cut into lardons layer.

Carefully pour the 4 eggs and 1 ¼ cups heavy cream custard over the 6 oz thick-cut bacon cut into lardons and 1 cup Gruyère cheese freshly grated. Pour slowly to avoid overflow — the filling should come to just below the crust rim. Transfer the quiche to the oven carefully.

Bake at 375°F for 30–35 minutes, until the edges are set and the center has just a slight wobble when gently shaken (like Jell-O, not liquid). The top should be lightly golden. Do not overbake — the custard will continue to set as it cools.