Preheat oven to 375°F (190°C). Grease 12‐cup muffin tin; set aside. Whisk together eggs, milk, salt and pepper. Stir in cheese and green onions; divide evenly among muffin cups.
Bake until eggs are set, approximately 12 to 15 minutes. Run thin knife around edge of each cup and remove frittatas. Let cool for 5 minutes or serve at room temperature. To serve, top with salsa.