Preheat the Oven
Heat your oven to 350°F (175°C). Lightly grease a 12-cup muffin tin with cooking spray or oil.
Prepare the Filling
In a bowl, combine the diced ham, shredded cheese, and chopped vegetables. Mix gently and distribute the mixture evenly among the muffin cups.
Whisk the Eggs
In a separate bowl, whisk together the eggs, milk (if using), salt, and pepper until smooth and well combined.
Pour the Egg Mixture
Carefully pour the egg mixture into each muffin cup, filling them about ¾ full to allow room for rising.
Bake
Place the muffin tin in the oven and bake for 18–22 minutes, or until the mini frittatas are set in the center and lightly golden around the edges.
Cool and Serve
Let them cool for a few minutes in the pan, then use a butter knife to gently loosen and remove each frittata.