Peel and cut the potatoes in small pieces. Boil in water for maximum 10 minutes. Drain away the water.
Preheat the oven at 225 degrees Celsius.
Butter an oven form and pour the potatoes and the corn in the form.
Grate the mozzarella cheese and pour over the potatoes.
Stir together the milk, eggs and spices and pour in the oven form.
Divide the tomatoes in half and add on top of the omelet.
Place the omelet in the middle of the oven for 20-30 minutes.