Preheat the oven to 350 degrees.
Bring a pot of water to boil. Once boiling, sprinkle in some salt then add in the pasta shells. Cook for 10 minutes, then drain.
While the pasta is boiling, heat up a small sauté skillet with the teaspoon of olive oil. Add in the diced sausages, onion, and green bell pepper. Season with a dash of salt and pepper. Cook for 5 minutes—the vegetables should be soft.
Remove the sausage and vegetable mixture to a large bowl. Add in the cooked pasta, pepperoni slices, the marinara sauce, and 1/2 of the fresh mozzarella. Mix until combined.
Move the sausage & pasta mixture to the casserole dish. Add dollops of the ricotta cheese to the top and sprinkle with the other 1/2 of fresh mozzarella.
Bake in the oven for 25 minutes. The cheese should be melted and the sauce should be bubbling.
Sprinkle with fresh cracked pepper and red pepper flakes, if desired. Serve warm.