Preheat oven to 350F (180C) and grease a 24 mini muffin pan or a 12 muffin pan.
Combine:
In a large bowl, whisk eggs and milk. Add in chopped spinach, shredded cheddar, chopped bacon, and pepper. Give it a quick mix to combine all ingredients.
Distribute egg mixture evenly into muffin pan cups.
Bake:
Bake in the preheated oven for 15-18 minutes. If you use a regular-size muffin pan (for 12 muffins), bake them for 26 to 28 minutes.
Serve:
Once cooked, allow the mini quiches to cool in the pan before carefully removing with a small knife or spatula.