Drain, wipe clean then dice the mozzarella.
Cut the tomatoes in half. Roughly chop the basil.
Beat the eggs with 2 Tbsp of milk. Add the mozzarella cubes and the pesto (or most of the basil leaves).
Heat the oil over medium heat in a large, heavy based frying pan. Pour in the egg mixture, then scatter the tomatoes over it. Season to taste. Turn down the heat.
When the top of egg is like gelatine, put it under the grill (broiler) to set. When it's golden brown, take it out of the oven and, using a spatula, transfer the frittata to a plate.
Let it cool, then cut it into wedges or squares and serve.