1. Preheat the oven to 350°F. Cook the pasta according to package directions until al dente, drain and set aside. Meanwhile, finely chop the onion. Heat the oil in a frying pan and fry the onion for 1 minutes. Add the beef and cook for 5 minutes. Add the tomatoes and crush finely with a wooden spoon. Add the spinach bit by bit until all is wilted and the liquid has evaporated.
2. Season with salt (in moderation) and pepper. Meanwhile, cut the mozzarella into pieces. Toss the pasta with the mozzarella and tomato sauce mixture. Spoon into a baking dish and bake for about 20 minutes on the middle rack of the oven.
Per serving: 563 calories, 23g fat, 10g saturated fat, 82mg cholesterol, 713mg sodium, 55g carbohydrate, 5g fiber, 8g sugar, 34g protein