Preheat oven grill to 350˚F.
Beat the eggs, milk, salt and pepper together in a medium size bowl. Place a 10-inch cast iron or non-stick skillet over medium high heat. Add the olive oil to the pan and swirl to coat the surface. Once hot, pour in the egg mixture. Working swiftly, use a spatula and stir the contents while shaking the pan for the first 10-15 seconds. This helps to cook the eggs evenly. Turn the heat to low and leave to cook for 2-4 minutes. Once you see the center is still shiny but almost set, take off the heat. Spoon the tomato sauce all over. Top with torn pieces of mozzarella and the prosciutto. Place under the grill for 1-3 minutes, or until the mozzarella has melted and the eggs are fully cooked. Remove from the oven.
Allow the frittata to sit in the pan for 5 minutes. To serve, transfer to a chopping board and slice into wedges. Top with some fresh basil leaves. Enjoy while warm.
For a dairy free alternative, simply substitute water for the milk. Omit the mozzarella or replace with a dairy-free topping of your choice.