Preheat the oven to 190°C.
In a medium bowl, whisk together the eggs, egg whites, milk, and lemon juice.
Heat the olive oil in a 20-cm nonstick ovenproof frying pan over medium-high
heat. Add the onion, peppers, salt, and black pepper. Cook, stirring, until the
onion is tender, about 3 minutes. Stir the vegetables and cheese into the egg
mixture, then pour the mixture into the pan. Bake the frittata until just set in the
middle and the cheese has melted, 20 to 25 minutes. Let cool for 4 to 5 minutes,
then cut into four wedges and serve.