Preheat the oven to 350°. In a large ovenproof nonstick skillet, heat 1 tablespoon of the oil. Add the onion and cook over moderately high heat until golden brown, 10 minutes. Add the greens and cook until wilted.
Season the eggs with salt and pepper and whisk in the remaining 2 tablespoons of oil. Pour the eggs into the skillet and cook over moderate heat until the bottom and sides begin to set. Lift the sides of the frittata to allow the uncooked eggs to seep under. Continue cooking until the bottom is set and the top is still runny, 3 minutes. Sprinkle the Parmigiano-Reggiano on top.
Transfer the skillet to the oven and bake for about 8 minutes, until the center of the frittata is set. Slide the frittata onto a cutting board. Cut into 1 1/2-inch squares and serve hot, warm or at room temperature.
One Serving 245 cal, 7 gm carb, 17 gm fat, 4.5 gm sat fat, 17 gm protein, 3 gm fiber.