Sausage, Mozzarella, and Broccoli Rabe with Shells

Preheat oven to 400 degrees. In a large skillet, heat oil over medium-high. Add onion and thyme; season with salt and pepper. Cook, stirring occasionally, until onion is golden brown, 15 minutes (reduce heat if browning too quickly).

Add garlic and sausage. Cook, breaking meat up with a wooden spoon, until browned, about 5 minutes. Add flour and cook, stirring constantly, 30 seconds. Add tomatoes, breaking them up with spoon. Cook sauce until slightly thickened, 5 to 8 minutes. Season with salt and pepper. Discard thyme.

Meanwhile, in a large pot of boiling salted water, cook pasta 4 minutes less than package instructions. Add broccoli rabe to pot and cook 15 seconds. Drain pasta and broccoli rabe and return to pot. Stir in sausage mixture. Transfer to a 3-quart baking dish or divide among four 16-ounce gratin dishes. Top with mozzarella and Parmesan. Bake until cheese has melted and liquid is bubbling, about 15 minutes.