Preheat the oven to 350 degrees. In a large bowl, combine the egg whites, water, and cream of tartar. Using an electric mixer on high speed, beat until stiff peaks form.
In a small bowl, beat the egg yolks, _ teaspoon salt and a pinch of pepper until pale and thickened. Stir in the parmesan cheese, scallions, chives and parsley. Gently fold in the egg white mixture.
Place a ovenproof skillet over medium-high heat and add butter. When the butter has foamed and the foam begins to subside, tilt the pan to distribute the butter evenly. Scoop the egg mixture into the pan and smooth the surface gently with a nonstick spatula. Reduce the heat to medium and cook the omelet until the bottom is golden, about 5 minutes. Place the pan in the oven and bake until the omelet is firm, about 7 minutes longer.
Remove the pan from the oven and, using a spatula, loosen the edges of the omelet, then slide it onto a platter. Cut the omelet in half and serve. Makes two servings.