Depending on the size of your crust you will want to make a 4 egg or 5 egg quiche. Ingredients are listed for a 4 egg quiche which is good for 9" pie crusts that aren't super deep or have shrunk a bit during the par baking. If you find your filling doesn't reach high enough in the pie shell you can mix an egg and 1/4 cup of half and half and add it to the unbaked filling already in the quiche to make a 5 egg quiche.
Preheat oven to 350F
Sauté the diced onions in 2 tsp of melted butter until soft, about 6 minutes. Set the onions aside.
2/3 cup diced onion, 2 tsp butter
In a large bowl, add the eggs, half-and-half, salt, pepper, and paprika. Whisk together until well combined. Stir in the cheese, sauteed onions, and mix-ins (leaving out ¼ cup, which will be added on top later). Pour this egg mixture into the cooled par-baked pie shell. Sprinkle with paprika if using.
4 large eggs, 1 cup half and half, ¼ teaspoon salt, ¼ teaspoon pepper, ¼ teaspoon paprika, 1 cup shredded or crumbled cheese of your choice, 1-1 ½ cups of add-ins
Place the quiche on a metal baking sheet if you have one, and bake for 30-50 minutes, or until the top begins to solidify but not all the way. If you are making a smaller filling with 4 eggs check closer to 30 minutes. See notes for more info. Sprinkle the leftover ¼ cup of mix-ins on top and bake for another 15 minutes. The middle can be very slightly jiggly, but the rest should be set solid.
1 partially baked pie crust
Allow the quiche to cool for a minimum of 15 minutes before attempting to cut it. Quiche can be served hot, at or near room temperature, or cold.