How to keep my Frittata ingredients from separating

I just made some egg white frittatas in silicone muffin tins (recipe below). My question is, how can I keep the veggie mixture from rising to the top during baking? Thanks in advance.
- 32oz. Carton of Egg Whites
- ⅓ cup finely shredded mozzarella-part skim
- ½ of a 6oz block of gruyere -finely shredded.
- ⅓ of large slicing tomato
- ¼ of a large red bell pepper
- 1 to 1½ cups of baby spinach
- ½ tsp garlic powder
- ¼ tsp of onion powder
- ¼ tsp salt
- ¼ tsp pepper
Directions
- Preheat oven to 350°
- Puree tomato, pepper and spinach in food processor
- Add all ingredients to large bowl
- Place greased silicone muffin tin on large cookie sheet
- Scoop egg mixture into muffin tins, being sure to keep mixture stirred for consistency
- Bake frittatas for approx 25 minutes
Bryant Garbutt Asked question