Preheat the oven to 200°C.
Heat the oil in a 28cm frying pan and gently cook the onion and garlic until just tender. Add the mushrooms and pepper to the pan and cook over a moderate heat for 5 minutes, turning at times.
Whisk together the eggs, oregano, and milk, season with salt and pepper. Pour the eggs into the frying pan and cook for a few minutes before arranging the pepper, salami and olives over the surface and then sprinkle over the grated parmesan and cheddar cheese.
Continue to cook until the edges pull away from the side of the pan and then transfer to the oven to cook until the frittata is just set, 5-10 minutes.
Serve with steamed greens or a crisp green salad.