In a large pot or Dutch oven, heat olive oil over medium heat. Add diced onion and cook until softened, about 5 minutes.
Add ground beef and cook, breaking it up with a spoon, until browned. Drain off any excess grease.
Add diced bell peppers and garlic to the pot and cook for another 3-5 minutes, until slightly softened.
Pour in beef broth, diced tomatoes, and tomato sauce. Stir in Italian seasoning, red pepper flakes, salt, and black pepper.
Bring the mixture to a boil, then reduce heat to low, cover, and simmer for 20-25 minutes, or until the rice is cooked through.
Serve hot, garnished with shredded cheddar cheese if desired.