How to Make It
In a large heavy skillet, heat the oil over medium high heat until hot, then stir in the sausage, breaking it up with a spatula, and cook, until golden, about 6 minutes. 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. Stir in the pepperoni and season the sauce with salt and pepper to taste.
In a large pot of boiling salted water, cook the spaghetti until al dente, then drain. Toss the pasta with the sauce and serve topped with the cheese.