In a large pot, heat the oil over medium heat and add the onion, garlic, and red pepper flakes. Saute for 2 minutes, then add the beef broth and 3 cups of water. Add all of the remaining ingredients, except the tortellini. Bring to a boil, then reduce the heat and let simmer for 5 minutes. Add the tortellini and simmer until the tortellini are cooked, another 5 minutes. Season with salt and pepper, to taste. Ladle into serving bowls and serve with Herb Focaccia, if desired.