Preheat the oven to 450°F (230°C). Place pie pastry into a 9-inch pie pan, line with aluminum foil and pie weights or dried beans.

Bake the crust until the edges are golden, about 8 minutes. Remove the foil and weights to continue baking until set and dry, about 5 minutes more. Reduce oven temperature to 325°F (165°C).

Cook bacon in a skillet over medium-high heat until crispy, about 10 minutes. Drain on a paper towel and crumble once cool.

Cook onions in the bacon drippings over medium heat until tender, about 5 minutes. Remove using a slotted spoon.

Toss shredded Swiss cheese with flour. Whisk milk, eggs, and salt together in a large bowl, then mix in bacon and onions.

Combine the milk mixture with the cheese mixture. Pour into the prepared pie crust.