Wash and clean the zucchini and cut them into slices about 1 cm thick. Peel onion and garlic and dice finely. Wash the rosemary, shake dry and chop the needles very finely. Coarsely grate the mozzarella.
Whisk eggs, milk and pesto. Season with rosemary, salt and pepper. Heat the oil in portions in an ovenproof pan (approx. 28 cm Ø). Fry the zucchini in the oil for approx. 4 minutes, turning them over.
After about 2 minutes add onion and garlic. Preheat oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer).
Arrange the zucchini in the pan. Pour egg milk over it, sprinkle with cheese. Leave in the oven for about 20 minutes. Remove from the oven, leave to rest for about 5 minutes and lift out of the pan. Cut the frittata into cubes.
Olives or finely sliced salami taste good with it.