Preheat oven to 350°F. In a large oven-safe skillet over medium-high heat, cook sausage, breaking up with a wooden spoon, until seared and no longer pink, about 4 minutes. Add onion, garlic, and bell pepper and cook, stirring, until soft, 5 minutes more.
Add pasta and stir until coated, then pour over marinara and broth and season with salt. Bring liquid to a boil, then reduce heat to medium-low and simmer until pasta is al dente and almost all liquid has been absorbed, about 25 minutes.
Stir in half-and-half and simmer until mostly absorbed, 2 minutes more. Remove from heat and stir in half of mozzarella.
Transfer mixture to a large baking dish and sprinkle with remaining mozzarella. Bake until cheese is bubbly and golden, about 10 minutes.