Begin by combining the flour and a pinch of kosher salt in a bowl. Add the cold butter, and using your hands or a pastry cutter, mix until it resembles coarse crumbs. Next, gradually add ice-cold water, mixing gently until the dough comes together. Form it into a disk, wrap it in plastic, and refrigerate for at least 30 minutes. This step helps the butter solidify and produces a flaky crust.

While the dough chills, preheat your oven to 375°F (190°C). This ensures the crust bakes evenly and crisps up nicely.

In a skillet over medium heat, melt the tablespoon of butter. Add the diced bacon and cook until crispy. Remove bacon and drain on paper towels. In the same skillet, add the sliced onions and sauté until soft and slightly caramelized. Stir in the fresh thyme and season with salt and pepper.

Once chilled, on a floured surface, roll out the pastry dough to fit your pie or tart pan. Drape the dough into the pan, pressing it gently into the corners. Leave a bit of overhang as it will shrink during baking.

In a large bowl, whisk together the eggs, heavy cream, Dijon mustard, and freshly cracked black pepper. Add the cooked bacon, sautéed onions, and shredded Gruyère cheese, mixing gently.

Carefully pour the filling mixture into the prepared crust. Bake for about 35-40 minutes or until the center is set, and the top is golden brown. It should puff slightly during baking—the hallmark of a properly cooked quiche.

Once you remove it from the oven, allow the quiche to cool for at least 15 minutes before slicing. This waiting period helps the custard firm up a bit more.