Heat oil in a large saucepan over medium-low heat. Add onion and garlic and cook until onion is slightly tender, about 5 minutes. Stir in tomato paste, cumin, chili powder, broth, and cilantro. Bring to a boil. Reduce heat, cover and simmer for 15 minutes. Add tortillas, tomatoes, beans, and zucchini. Cover and simmer until zucchini is tender, about 5 minutes. Stir a dollop of sour cream into each bowl of soup at the table. Tip: You can freeze leftover tomato paste in tablespoon size portions, wrapped in plastic wrap, in a freezer bag. That way you won?t have to waste the rest of the can every time you need a tablespoon or two.