Preheat the oven to 350°. Bring a medium saucepan of water to a boil. Add the potatoes and cook until just tender, about 5 minutes. Drain and pat dry.
In a large, ovenproof nonstick skillet, heat the olive oil. Add the pepper and cook over high heat until just softened, about 3 minutes. Add the potatoes and cook, stirring occasionally, until lightly browned, about 4 minutes. Season with salt and pepper.
In a large bowl, beat the egg whites with the milk, scallions and cheese. Add the egg mixture to the skillet and cook over high heat just until the bottom begins to set, about 3 minutes. Transfer the skillet to the oven and bake in the upper third for about 15 minutes, until the egg whites are set. Slide the frittata onto a plate, cut into wedges and serve.