Preheat the oven to 350°F (175°C).
Cook the jumbo pasta shells in a large pot of salted boiling water according to package directions until al dente; drain in a colander and spread shells on a large plate to cool so they don’t stick.
In a large skillet over medium heat, brown the ground beef and pork sausage, breaking up meat while cooking until no pink remains; drain excess grease.
Transfer the cooked meat to a large mixing bowl. Stir in chopped bell pepper, chopped onion, 1/2 cup freshly grated Parmesan, dried oregano, minced garlic, sliced olives, and 1/2 cup of the marinara sauce. Season with salt and freshly ground black pepper to taste.
Spoon a heaping spoonful of the meat mixture into each cooled jumbo shell and arrange the filled shells in a single layer in a 9x13-inch baking dish lightly spread with a thin layer of marinara sauce.
Pour the remaining marinara sauce evenly over the filled shells. Scatter chopped pepperoni over the top, then sprinkle with shredded mozzarella cheese.
Bake in the preheated oven for 25–30 minutes, or until the cheese is melted and bubbly.
Remove from oven and let rest a few minutes before serving.
