Set oven to 350°F. Line a 12-cup muffin tin with silicone liners or spray a silicone pan with cooking spray.
Combine egg whites, cottage cheese, shredded cheese, feta, salt, and pepper in a blender. Blend 30 seconds until mostly smooth. Small cheese chunks are okay.
Mix spinach, roasted red peppers, and green onion in a small bowl.
Divide vegetable mixture among muffin cups. Pour egg mixture over veggies, filling each cup about ¾ full.
Place muffin pan on a rimmed baking sheet. Fill sheet halfway with boiling water and carefully transfer to oven.
Bake for 30–35 minutes until set and lightly firm. Cool 5 minutes before removing from the pan.