- Melt the butter in a large heavy frying pan. Cook the sliced onions in the butter over moderate heat, until they are evenly browned.
- Add the sliced potatoes, stir well, cover the pan and cook for 15-20 minutes, stirring occasionally. After 10-15 minutes, add the zucchini or other vegetables, so they will be cooked at the same time as the potatoes.
- Beat the eggs with the water and half the cheese. Pour this mixture over the pressed-down vegetable mixture, and cook over gentle heat for 10 minutes or until the sides and bottom are cooked.
- Sprinkle the remaining parmesan over the top, then place under a grill until the top puffs up and browns lightly. Serve hot, warm or at room temperature, with a salad if desired.
Variations: Add chopped garlic with the potatoes; and fresh or dried herbs with the egg mixture.