Preheat the grill in the oven. Heat the oil in an ovenproof, non-stick pan and fry the onions and garlic until the onions are soft. Then add the chard and the peppers cut into pieces and fry for about 3 minutes until the chard is soft. In the meantime, mix the eggs, egg white, salt and pepper in a bowl and stir in the mozzarella. Pour the whole thing over the vegetables and stir gently. Cook on low heat for about 7-8 minutes, then place in the oven under the grill for about 2 minutes. Take out of the oven and let rest for about 2 minutes before serving. Per serving (1/4 of the frittata) approx. 233g - 206 Kcal.