Open and unfold one pie crust, then press out any fold lines. With a 2-1/2 inch biscuit cutter, cut out 12 rounds. Repeat with remaining 3 pie crusts.
In the prepared mini muffin tins, press 1 round of pie crust into the bottom and up the sides of the cup.
In a 4-cup measuring cup, combine the eggs, milk, and the salt. Whisk well. Set aside.
To assemble the Mini Bacon Quiche:
In 12 of the mini muffin cups, add 1 tablespoon of cheese. Evenly divide the chopped bacon among the 12 cups, using about 1/2 teaspoon bacon in each. Top with a couple slices of green onion.
To assemble the Mini Ham and Cheese Quiche
In 12 of the mini muffin cups, add 1 tablespoon of cheese. Evenly divide the chopped ham among the 12 cups, using about 1/2 teaspoon ham in each.
To assemble the Mini Spinach Quiche
In 12 of the mini muffin cups, add 1 tablespoon of cheese. Evenly divide the wilted spinach among the 12 cups, using about about 1/2 teaspoon spinach in each. Evenly divide the chopped red pepper among the mini muffin cups.
To assemble the Mini Mushroom Quiche
In 12 of the mini muffin cups, add 1 tablespoon of cheese. Evenly divide the chopped mushroom among the 12 cups, using about 1/2 teaspoon mushroom in each.
To finish and bake the mini quiche:
Pour egg mixture into each of the 48 cups, filling to within 1/4 inch of the top. Bake at 375 degrees until golden brown, about 25 to 30 minutes. Cool slightly and serve warm.
Pie crust: Get a head start with frozen and thawed store-bought pie crust, or make your own pie crust from scratch. To make the crust in advance, make the dough, form it into a disk shape, wrap it tightly in plastic wrap and foil, and freeze for up to 2 months. To use, thaw it overnight in the refrigerator, then roll out the crust before slicing with a biscuit cutter.
Bacon: To crisp up a batch at once, I like to fry bacon in the oven. To do so, preheat the oven to 400 degrees. Line a rimmed baking sheet with foil for easy clean up. Set a rack on top if using. Arrange the bacon in a single layer. Bake until bacon is crispy (start checking at 10 minutes; I usually bake it for about 15 minutes).
Swiss cheese: Pre-shredded cheese is tossed with agents to prevent the cheese from sticking together, but it should still melt and thicken up fine. For the best consistency, grate your cheese fresh from a block.
Spinach: To wilt the spinach, place spinach into small saucepan and cover with water. Bring to a boil and cook until spinach is dark green and wilted. Alternatively, in a small saucepan, place the spinach with 1/4 cup of water and sauté over medium high heat until wilted. Place spinach between two kitchen towels or paper towels and remove as much water as possible.
Yield: Each of these Mini Quiche recipes creates one dozen of each of the four quiche varieties; 48 bite-sized mini muffin tin pieces total.
Storage: Store leftovers covered in the refrigerator for up to 4 days.
Freezer: Assemble, bake, and cool the Mini Quiches. Freeze the egg cups on a plate or baking sheet until they are solid, then transfer them to zip-top freezer bag (so they don't stick together).
Reheat: Bake the frozen mini quiche straight from the freezer. Transfer to a baking sheet and reheat in a 400-degree oven for 5 to 10 minutes.
Crust swap: Feel free to substitute store-bought phyllo dough for the pie crust, if desired. Slice the same shape and at the same temperature as the pie crust. When reheating from frozen, reduce oven temperature to 220 degrees and reheat for about 20 minutes to prevent the phyllo dough from burning.
Quiche muffins: Slightly larger than the mini quiche, these quiche are baked in a regular-sized muffin tin instead of mini muffin tin.
Egg muffins: Crustless quiche baked in a regular-sized muffin tin.