Place potatoes on a microwave-safe dish and microwave until just able to pierce with a fork. Allow to cool until able to handle. Cut potatoes into 1-inch chunks; set aside.
In a large, nonstick skillet, over medium-high heat, melt oil and butter. Add green onion and garlic; cook briefly just until onion begins to color. Add cooked potato chunks. Using a spatula, mix gently, turning often until potatoes begin to brown. Add salt and pepper to taste.
In a mixing bowl, combine eggs and water; whisk thoroughly. Add Parmesan cheese and dill. Pour egg mixture over vegetables in skillet; continue to cook, pushing eggs into center from edge of pan. When eggs have cooked and are set, place slices of Fontinella cheese over eggs to cover top of frittata. Turn off heat and cover pan. Allow time for cheese to melt.
Carefully slide frittata onto serving dish and cover with tomato slices. Serve immediately. Makes 6 servings. Enjoy!