Preheat oven to 375 F. In large bowl, whisk together eggs, yogurt, pesto, salt, and red pepper flakes (if using). Stir in cooked chicken, broccoli, and sun-dried tomatoes. Divide among 12 standard-sized greased muffin cups and bake until eggs are set, about 20 minutes. Let frittatas cool for 5 minutes before unmolding.