Heat the oven to 400°F and coat a 9-by-13-inch baking dish with cooking spray.
Combine the dry shells, sausage, pepperoni, tomato sauce, broth, oregano and salt directly in the dish.
Press the shells below the liquid, cover tightly with foil and bake for 40 minutes.
Stir well and bite into a shell from the center; if it is still chalky, add 1/4 cup hot water, reseal and bake for 5 to 10 minutes more.
Top the tender shells with mozzarella and bake uncovered for 8 minutes, until melted and browned at the edges.
Let the casserole stand for 8 minutes before scooping.