Preheat the oven to 350° F. Arrange potatoes in a 9-inch pie plate, along with onion and sage.
Pour in eggs and bake in the middle of the oven until set, about 20 minutes.
Immediately sprinkle on Parmesan. Cut into wedges and serve warm for breakfast, brunch, lunch, or with soup and salad for a light dinner.
Servings: 4