Pre-heat your oven to 180C/350F/Gas Mark 4, and brush a 27x18cm rectangular dish with oil
Pre-cook the vegetables in a pan with a little oil. Fry them until they are soft
Whisk together the eggs, egg whites and semi-skimmed milk. You can season with salt and pepper at this point to your liking
Tip the vegetables into the dish, then pour in the egg mixture. Sprinkle the parmesan cheese over the top
Bake the omelette for 35 minutes until it is golden and doesn't have a wobble to it anymore
Slice into portions and eat hot or cold