1. Preheat the oven to 350°F. Lightly oil a 9x13x2–inch baking dish.
2. Heat the olive oil in a heavy pot over medium-low heat. Add the sausages and cook, breaking them up into clumps, until browned, 10 to 12 minutes. Using a slotted spoon, transfer to a large bowl.
3. Meanwhile, cook the pasta in a large pot of boiling salted water until just tender, about 12 minutes. Drain well and add to the sausages along with the marinara sauce, tomatoes, basil, oregano, pepper flakes, diced mozzarella, salt, and pepper. Toss well. Transfer to the prepared baking dish and cover the top with the mozzarella slices.
4. Bake until the cheese is melted and the pasta is heated through, about 20 minutes.
Serves 6. Per serving: 630 calories, 59g carbs, 32g protein, 29g fat, 65mg cholesterol