Preheat your oven to 375°F (190°C). While it preheats, cook the bacon in a large skillet over medium heat until it’s nice and crispy. Set it aside on a paper towel lined plate to cool.
Dice the onion and spinach. Once the bacon is cooled enough to handle, cut it into bite-sized pieces.
Crack the eggs into a large bowl. Add the milk, salt, and pepper. Whisk it all together until fully combined.
Spray or drizzle a muffin tin well with oil. Add the spinach, onion, and bacon in each well, then pour the egg mixture on top, until each is filled to about 80%.
Bake for 18-22 minutes, or until the eggs are cooked through.
