Preheat the oven to 350 degrees F.
Heat a large (12-inch), nonstick skillet with an ovenproof handle over medium-high heat. When the pan is hot, add the oil and onions and cook for 3 minutes, until wilted. Add the garlic and crumbled meatballs and cook for 2 minutes. Melt the butter in the skillet and add the spaghetti. Toss to coat well.
In bowl, beat the eggs with the cream until slightly foamy. Season with salt and pepper. Pour the eggs into the pan. Stir the eggs gently until they start to thicken, about 1 1/2 to 2 minutes. Stop stirring and allow the eggs to set up on the bottom, about 1 minute.
Sprinkle the cheese over the top of the frittata and transfer the skillet to the oven. Bake for 12 to 15 minutes or until set and golden brown. Place the frittata on plate. Garnish with fresh chopped parsley and serve immediately alongside of warmed, leftover tomato sauce.
Yield: 4 to 6 servings