Bake in a miniature muffin pan.  They taste great hot or at room temperature, so you can make them in advance. 
Serves 8; 3 frittatas per serving
Cooking spray
½ cup finely chopped onion
2/3 cup chopped reduced-fat ham (about 2 ounces)
1/3 cup (about 1 ½ ounces) shredded reduced-fat extra sharp cheddar cheese
2 tablespoons chopped fresh chives
1/8 teaspoon dried thyme
1/8 teaspoon black pepper
4 large egg whites
1 large egg
Preheat oven to 350°.

Heat a large nonstick skillet with cooking spray over medium-high heat.  Add onion; sauté 2 minutes or until crisp-tender.  Add ham; sauté 3 minutes.  Remove from heat; cool 5 minutes.  Combine remaining ingredients in a large bowl; stir with a whisk.  Add ham mixture, stirring with a whisk.  Spoon mixture into 24 miniature muffin cups coated with cooking spray.  Bake a 350° for 20 minutes or until set.