In a large skillet over medium heat, cook the ground beef and Italian sausage until browned, breaking it apart with a spoon. Drain any excess fat.
Add the diced onion, green bell pepper, and minced garlic to the skillet. Sauté until the vegetables are soft, about 5 minutes.
Stir in the diced tomatoes, tomato sauce, tomato paste, sugar, Italian seasoning, salt, black pepper, and red pepper flakes. Bring to a simmer and let it cook for about 20 minutes, stirring occasionally.
While the sauce simmers, cook the spaghetti according to package instructions in a large pot of salted boiling water until al dente. Drain and set aside.
Once the sauce has thickened, combine it with the cooked spaghetti in the skillet. Toss to coat the pasta evenly with the sauce.
Serve hot, topped with grated Parmesan cheese and fresh basil leaves if desired.