Spray a muffin tin generously with nonstick cooking spray
Distribute spinach, tomatoes, mushrooms, and bell peppers evenly into the 12 cups
In a large bowl, whisk together eggs, milk, salt, pepper, onion powder, garlic powder, and paprika until combined
Pour egg mixture evenly into the 12 tins, to cover the veggies
Sprinkle the tops of the muffin cups with sharp cheddar cheese
Bake the egg muffins at 350 degrees for 25 - 30 minutes, until eggs are cooked through
Allow to cool several minutes in the pan, then use a butter knife to loosen the edges before removing them
Store leftover egg muffin cups in the fridge for up to 4 days