Make the crust: In a food processor, combine the all-purpose flour, whole wheat flour, and salt. Add the cold, cubed butter and pulse until the mixture resembles coarse crumbs. Gradually add the ice water, 1 tablespoon at a time, and pulse until the dough just begins to hold together.

Transfer the dough to a lightly floured surface and shape it into a disk. Wrap in plastic wrap and refrigerate for at least 30 minutes.

Preheat the oven to 400°F (200°C).

On a lightly floured surface, roll out the chilled dough into a 12-inch circle, about 1/8 inch thick. Carefully transfer the dough to a 9-inch pie dish and press it into the bottom and up the sides. Trim any excess dough from the edges.

Line the crust with parchment paper or aluminum foil and fill it with pie weights or dried beans.

Bake the crust for 15 minutes, then remove the weights and liner. Bake for an additional 5-10 minutes, or until the crust is lightly golden. Remove from the oven and set aside.

Reduce the oven temperature to 375°F (190°C).

In a large bowl, whisk together the eggs, heavy cream, milk, salt, and pepper until well combined.

Sprinkle the Gruyère cheese, crumbled bacon, caramelized onions, and sautéed spinach over the pre-baked crust.

Carefully pour the egg mixture over the fillings, ensuring the ingredients are evenly distributed.

Bake the quiche for 35-40 minutes, or until the center is set and the top is lightly golden. (The quiche should still have a slight jiggle in the center.)

Allow the quiche to cool for at least 15 minutes before slicing and serving. Enjoy!