Preheat oven to 350°F. Spray a 12-count muffin pan generously with nonstick spray.
In a medium bowl, preferably one with a pour spout, whisk together the eggs, milk, salt, pepper, and onion powder just until combined.
Evenly distribute the chopped vegetables and cheese directly into the greased muffin cups.
Pour the egg mixture into each muffin cup, filling about ¾ full.
Bake for 15–20 minutes or until puffy and golden brown around the edges.
Remove from the oven and cool in the pan for 5 minutes.
Remove the egg muffins from the pan, and either serve immediately, or cool completely and store in the refrigerator or freezer.
