Mix ground beef, egg, rice, bread crumbs, and onion powder together in a large bowl.
Pack the ground beef mixture into the green bell peppers, eliminating all air pockets. Form the remaining beef mixture into large balls.
Combine water, tomato sauce, tomato paste, salt, black pepper, and garlic powder in a Dutch oven; bring to a boil. Carefully lower peppers and meatballs into the sauce with a slotted spoon. Bring back to a boil; reduce heat and simmer, partially covered, until beef is cooked through and rice is tender, about 1 hour.
Mix sour cream and flour together in a small bowl. Slowly stir in some of the sauce. Pour into the Dutch oven and mix well. Simmer until flavors combine, 3 to 4 minutes more.
