In a large pot over medium heat, cook and stir the ground beef until evenly brown; drain.
Mix the water, tomatoes, mushrooms, green bell pepper, onion, and garlic into the pot with the beef. Season with basil, salt, and pepper. Bring to a boil, reduce heat, and simmer 45 minutes.
Mix the feta cheese into the soup, and continue cooking about 15 minutes before serving.