Preheat oven to Gas Mark 6, 200°C (400˚F).
Place the prepared peppers, aubergine, onion, courgette, potato and garlic in a bowl and mix with a tablespoon oil. Season with a little salt and black pepper and then spread in a single layer in a roasting tray and roast for 15-20 minutes.
Line an oven proof baking dish (6cm deep, 20cm x 20cm), with baking parchment.
Transfer the vegetables to the prepared dish. Reduce oven to Gas Mark 4, 180°C (350˚F). Whisk the eggs and milk together in a jug. Add ⅔ of the cheese and the parsley. Season with a little salt and black pepper then pour the egg mixture over the vegetables. Shake the dish gently to allow the egg to spread to the base, sprinkle over the remaining cheese.
Bake the frittata for 20 minutes or until top is golden and the centre is set. Let it rest for a couple of minutes before cutting into squares and serving with some salad and crusty bread.