Place the vegetable mixture in a pan without defrosting and fry for approx. 3 minutes, turning. Add vegetable stock and sour cream and bring to the boil. Grate cheese and stir into the vegetable mixture.
Season with salt and pepper. Beat the eggs and milk and season with salt and pepper. Heat the fat in a frying pan. Bake 4 omelettes from the egg milk one after the other at low heat. Keep warm.
Arrange omelettes on plates, fill with vegetables. Coarsely crush peppercorns and sprinkle over the omelettes. Garnish with parsley and tomato slices.