- Melt the butter in a large heavy pan. Cook the onion in the butter over a moderate heat until evenly browned. Add the sliced, unpeeled 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 when the potatoes are.
- Beat the eggs with the water and half the cheese. Pour 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 brown 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.