Heat a large saucepan over medium heat. Crumble sausage into the pan; add onion and bell pepper. Cook and stir until sausage is completely browned, 7 to 10 minutes. Drain grease.

Add marinara sauce and tomatoes to sausage mixture; stir in garlic powder. Bring to a simmer, reduce heat to medium-low, and cook until tomatoes break down into sauce, 30 to 45 minutes.

Meanwhile, preheat the oven to 350 degrees F (175 degrees C).

Bring a large pot of lightly salted water to a boil. Cook frozen tortellini in boiling water until they float to the top and the filling is hot, 6 to 7 minutes; drain.

Gently fold tortellini into simmering sauce; cook for 5 minutes. Transfer tortellini and sauce to a 9x13-inch baking dish; sprinkle with mozzarella cheese.

Bake in the preheated oven until cheese is golden brown, 20 to 30 minutes.