Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 12-cup muffin tin with cooking spray or olive oil.
Prepare the Egg Mixture: In a large mixing bowl, crack the eggs and whisk them together with the milk until well combined. Season with salt and pepper to taste.
Add the Fillings: Stir in the chopped vegetables, shredded cheese, and cooked bacon or sausage (if using) into the egg mixture until evenly distributed.
Fill the Muffin Tin: Pour the egg mixture evenly into the prepared muffin tin, filling each cup about 3/4 full.
Bake the Egg Muffins: Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the egg muffins are set and slightly golden on top.
Serve and Enjoy: Remove the egg muffins from the oven and let them cool in the muffin tin for a few minutes. Use a butter knife to loosen the edges, then transfer the muffins to a wire rack to cool completely. Serve warm or at room temperature, and enjoy these delicious and protein-packed breakfast egg muffins!