Brown the Beef: In a large pot, heat the oil over a medium flame. Add the ground beef and cook until it's no longer pink, about 3-5 minutes, breaking it apart as it cooks. Drain off any extra fat and put the beef aside.
Cook the Vegetables: To the same pot, add the diced onion and both bell peppers. Cook them until they soften, stirring now and then, for about 3-4 minutes.
Add Flavor: Next, incorporate the garlic, Italian seasoning, and red pepper flakes into the pot, cooking until the mixture smells aromatic, roughly 1 minute.
Mix in Tomatoes and Broth: Add the diced tomatoes, tomato sauce, beef broth, and the browned beef back into the pot. Season with salt and pepper according to your taste preferences. Bring the soup to a boil, then lower the heat to simmer, letting it cook until the flavors meld, about 15-20 minutes.
Incorporate Rice and Parsley: Stir the cooked rice and fresh parsley into the soup, warming through for a couple of minutes.
Ready to Serve: Once everything is heated through, the soup is ready to be served. Enjoy your delicious Stuffed Pepper Soup immediately.