Bring large pot of water to boil; salt it. Heat oven to 400 degrees. Distribute meat in large skillet over medium high heat and cook, undisturbed, until browned on one side, about 5 minutes. Stir, cook another 2 minutes undisturbed, then add the onion and garlic, if using. Lower heat to medium and continue to cook, stirring occasionally, until vegetables are soft; add tomatoes and bring to a boil. Simmer while cooking pasta, stirring and seasoning with salt and pepper to taste; do not let sauce become too thick.

Cook pasta until just tender; it should still be too hard to eat. Drain it (do not shake the colander; allow some water to cling to the noodles) and toss it with the sauce and half the mozzarella. Grease a 9x13-inch baking dish and spoon mixture into it. Top with remaining mozzarella and the Parmesan if using. Bake until top is browned and cheese bubbly, 20-30 minutes.

From: Mark Bittman