Heat the oil in the cast iron casserole to medium heat, add the tomatoes, onion and garlic. Fry for 5 minutes and then place all these vegetables in a blender and blend very well. Transfer the sauce back to the casserole and continue frying for 10 minutes to low heat.
Then add the vegetable stock and season with salt. Add the double concentrate tomato purée and cook for 15 minutes.
Then start adding the vegetables. Add the potatoes, carrots and rice first and cook for 10 minutes. Then add the green beans and continue cooking for 5 more minutes.
The soup is ready when the potatoes and carrots are soft. Serve in bowls and warm some corn tortillas, slice an avocado and make your favourite spicy salcita.
Roll up some tortillas stuffed with avocado slices, mayo and spicy salcita, squeeze some lime juice onto the soup and enjoy.
*You can add chicken stock if you want to make it more substantial.