How to Make It
In a large heavy skillet, heat the oil over medium high heat until hot, then stir in the garlic and onion and cook until the vegetables are pale golden, about 5 minutes. Stir in the tomatoes with their juice and 1/2 teaspoon each salt and pepper. Cook, stirring occasionally, until the sauce is slightly thickened, about 15 minutes. In a blender, puree the sauce until smooth, then season with salt and pepper to taste.
Preheat the oven to 375°F.
Bake the spaghetti casserole until the cheese is melted, about 10 minutes. Serve.