How to Make It
Preheat the oven to 375°. In a bowl, beat the eggs with the water and season with 3/4 teaspoon of salt and 1/2 teaspoon of pepper. Beat in the Gruyère and prosciutto.
Heat the olive oil in a large, nonstick ovenproof skillet. Add the potatoes and cook over moderately high heat, stirring occasionally, until tender and golden brown, about 7 minutes. Add the scallions and cook for 1 minute. Stir the egg mixture and add to the skillet. Stir to distribute the potatoes. Cook until the bottom is just set, about 3 minutes; lift the frittata to allow the uncooked eggs to seep underneath.
Bake the frittata for about 10 minutes, until nearly set in the center.
Preheat the broiler. Broil the frittata 8 inches from the heat for 1 minute, until the top is just beginning to brown. Cut the frittata into wedges and serve hot or at room temperature.