Lightly oil a 12-hole muffin tin.
Make the sauce. Heat the oil in a saucepan, add the onion and fry for 5 minutes, stirring occasionally, until softened and just beginning to brown. Add the garlic, if using, and the tomatoes and season with salt and pepper. Stir and simmer for 5 minutes until the tomatoes are soft. Purée in a blender or food processor until smooth, then pass through a sieve into a bowl and keep warm.
Heat the oil in a frying pan, add the onion and fry until softened and just beginning to brown. Add the courgettes, stir to combine and cook for 3–4 minutes until softened but not browned.
Beat together the eggs, milk, Parmesan and mint in a bowl, then stir in the courgettes. Season well and divide the mixture between the 12 sections of the prepared tin. Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for about 15 minutes until lightly browned, well risen and the egg mixture has set.
Leave the frittatas in the tin for 1–2 minutes, then loosen the edges with a knife. Turn out and arrange on 6 plates with the warm tomato sauce. Garnish with extra mint leaves, if liked.