Bring a saucepan of salted water to a boil and preheat oven to 200 degrees C.
Meanwhile, heat up a non-stick frypan and add the sausage meat. You probably won't need any oil. Break it up as it cooks. When browned, add the onion and garlic and cook until softened. Add tomatoes and their liquid and bring to a simmer. Season with some pepper and let gently simmer while cooking pasta.
Boil pasta in the water until just tender but not overcooked. Drain but don't rinse - keep a little of the pasta water.
Grease a casserole dish and add the wet pasta, sauce, and most of the mozzarella. Stir until well combined then top with remaining mozzarella. Bake until top is browned, about 30 minutes. Serve!