Bring a large pot of salted water to a boil. Cook the pasta according to package directions until al dente. Drain and set aside.
In a large skillet over medium heat, cook the ground beef until fully browned, breaking it apart as it cooks. Drain excess grease if needed.
Stir in the diced bell peppers and cook for 4 to 5 minutes until slightly softened.
Pour in the marinara sauce and mix well. Let it simmer for 5 minutes so the flavors come together.
Add the cooked pasta to the skillet and toss until everything is evenly coated.
Sprinkle mozzarella cheese over the top. Cover the skillet for 2 to 3 minutes until the cheese melts, or place under a broiler briefly if the skillet is oven-safe.
Serve hot, optionally with extra cheese on top.
Notes
Refrigerate leftovers in an airtight container for 3-4 days. Freeze in portion-sized containers for up to 2 months; thaw overnight in the fridge before reheating. For a baked version, transfer to a baking dish, top with extra cheese, and bake at 375°F (190°C) until bubbly and golden.