Heat the oven to 350°F, then place a silicone 12-cup muffin tray on a baking sheet and coat the cups with non-stick spray. The flexible cups should sit flat and remain stable on the sheet.

Blend the egg whites, mozzarella, cottage cheese, and feta for about 30 seconds until the mixture is smooth and foamy. Look for tiny bubbles across the surface rather than visible cottage cheese curds.

Scatter the diced roasted red pepper and chopped spinach among the cups, then pour the blended egg mixture over them and season with salt and black pepper. Fill the cups close to the top, leaving enough room for a gentle rise.

Set the baking sheet on the middle oven rack, then carefully pour boiling water into the sheet until it reaches about halfway up the silicone cups. The water should surround the cups without splashing into the egg mixture.

Bake for 30 to 35 minutes, then let the bites stand for 5 minutes before lifting them from the silicone mold. The tops should feel firm with a gentle spring when lightly pressed, and the bites will begin to pull away from the silicone edges.