Set oven rack about 6 inches from the heat source and preheat the oven's broiler.
Place sausage in a baking dish and cook, rotating every 5 minutes, until no longer pink in the center, about 20 minutes. An instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C). Cool to room temperature and slice. Reduce oven temperature to 350 degrees F (175 degrees C).
Place naan bread on a baking sheet; spread marinara sauce over naan and top with mozzarella cheese. Layer sliced sausage on top of cheese and spread onion and bell pepper slices over the sausage. Sprinkle Cajun seasoning over the top.
Bake in the oven until cheese is melted and toppings are hot, about 10 minutes. Let rest for 5 minutes before slicing.