Preheat oven to 350F (175C).

Prepare the pie shell.

Dice the bacon into small pieces.

Dice the onions.

In a skillet over medium heat, saute the bacon until mostly rendered.

Add the diced onions to the skillet and saute for 2-4 minutes, until softened.

Remove the bacon and onion mixture from the skillet and drain any excess grease.

In a mixing bowl, combine the milk and eggs.

Season the custard mixture with salt and pepper to taste.

Grate or shred the Swiss cheese.

Combine the Swiss cheese with the sauteed bacon and onions.

Place the cheese, bacon, and onion mixture loosely into the prepared pie shell.

Pour the custard over the cheese mixture, ensuring it is evenly distributed.

Bake the quiche in the preheated oven for approximately 40-45 minutes, or until the custard is set and the crust is golden brown.

Let the quiche cool slightly before serving.