Make the sauce. Heat the oil in a saucepan, stir in the onion and fry for 5 minutes, stirring occasionally until softened and just beginning to brown. Add the garlic, if using, the tomatoes and season with salt and pepper. Stir and simmer for 5 minutes until the tomatoes are soft. Purée in a liquidizer or food processor until smooth, 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, then stir in the courgettes. Season well and pour the mixture in the 12 greased sections of a deep muffin tin. Bake into a preheated oven, 190°C (375°F), Gas Mark 5, for about 15 minutes until they are lightly browned and well risen and the egg mixture has set.
Leave in the tin for 1–2 minutes, then loosen the edges with a knife. Turn out and arrange on plates with the warm tomato sauce. Garnish with extra mint leaves, if liked.