Preheat the oven to 350 degrees F. Fill a large pot with salted water and bring it to a boil. Add the jumbo pasta shells and cook them until al dente according to package directions. Drain and set the pasta aside to cool.

In a large skillet over medium heat, add the ground beef, diced onion, and diced bell pepper, then sprinkle in Italian seasoning, garlic powder, salt, and pepper. Cook until the beef is browned and the vegetables are soft, about 7–8 minutes. Add 2 cups of the pasta sauce and cook for another 2–3 minutes. Set the meat mixture aside.

Spread the remaining pasta sauce on the bottom of the baking dish. Fill each cooked shell with the meat sauce, then place the stuffed shells in the dish. Top with the remaining meat sauce and mozzarella cheese.

Cover the baking dish with aluminum foil and bake the dish for 20 minutes. Remove the foil and bake it for another 10–15 minutes, until the cheese melts and the casserole is warmed through.

Let the dish cool for a few minutes before serving.