Crack the eggs into a jug or bowl with the milk and a little salt and pepper. Whisk until light and fluffy and set aside.
Heat some calorie controlled spray oil over a medium heat in an ovensafe nonstick skillet, frying pan (10 - 12 inches). When hot, saute the chopped sweet peppers and scallion for 3 - 4 minutes.
Add the whisked egg to the pan and swirl around gently so that the egg mixture covers the chopped veg. Cook for a couple of minutes until the egg starts to set around the outside of the pan but is still liquid in the middle.
Tear up the mozzarella and dot it evenly over the top of the still wet frittata. Place the pan into the preheated oven for 10 - 12 minutes or until the eggs have set and the mozzarella starts to bubble.
Remove from the oven, sprinkle over some chopped chives if using and leave to cool for 5 minutes.