In a large bowl, chop the cooked vegetables into small pieces (if I'm using squash, I mush it down with a fork so that picky eaters won't notice!).
Whisk the eggs and parsley into the cooked vegetables.
In a small frying pan, heat the butter. When hot add the vegetable/egg mixture and cook on one side for 4-5 minutes.
Flip the omelet and place cheese on top. Cook for 2-3 more minutes.
Fold omelet in half, then cut in half and serve.