Heat a large pot over medium heat. Crumble sausage into the saucepan; add onion and green bell pepper. Cook and stir the sausage mixture until the sausage is completely browned, 7 to 10 minutes. Drain grease.
Pour marinara sauce and diced tomatoes over the sausage mixture; season with garlic powder and stir. Bring the mixture to a simmer, reduce heat to medium-low, and cook at a simmer until the diced tomatoes break down into the sauce, 30 to 45 minutes.
Preheat oven to 350 degrees F (175 degrees C).
Bring a large pot of lightly salted water to a boil. Cook the frozen tortellini at a boil until they float to the top and the filling is hot, 6 to 7 minutes; drain.
Gently fold the cooked tortellini into the simmering sauce; cook together another 5 minutes. Pour tortellini and sauce into a 13x9-inch baking dish; top with mozzarella cheese.
Bake in preheated oven until cheese is golden brown, 20 to 30 minutes.
