1. Place potato and sweet potato cubes in a pot with salted water. Cook over medium flame for 15 minutes, until the vegetables are partially softened. Drain water and transfer potatoes to a bowl.
2. Add remaining ingredients, except for the mozzarella cheese, and mix well.
3. Preheat oven to 400°F (200°C).
4. Pour potato mixture into an oven-to-table baking dish or divide into ramekins and bake for 20 minutes (a little longer if not using ramekins).
5. Remove from the oven, sprinkle with grated cheese, and bake for seven more minutes, or until cheese turns golden.