Toast tomatoes, onion and garlic in a frying pan, browning them on all sides.
Peel tomatoes and purée them in a food processor or liquidiser, together with the onion, garlic and 125 ml chicken stock.
Pass the liquid through a sieve to remove seeds.
Cut chillies into 1 cm wide rings and remove the seeds.
Heat oil in a large pot and fry chillies for about 1 minute, just to release the flavour and turn them crispy.With the oil still hot, add tomato purée, frying for 2 minutes.
Lower heat and simmer for about 5 minutes, stirring constantly, until the purée thickens slightly and changes colour.
Add the rest of the chicken stock and the epazote.
Bring to the boil, seasoning with salt and pepper to taste.
Cover and leave on medium heat for about 15 minutes to consolidate the flavours.
Meanwhile, cut tortillas in half and then into thin strips. (If the tortillas are fresh, cut them and dry them out for about an hour in a 150 °C preheated oven).
Fry in batches in about 1 cm cooking oil until golden brown and crispy, turning them at least once (about 3 minutes).
Place fried strips on absorbent paper to remove excess oil. Just before serving, heat soup.
Divide the tortilla strips between the bowls and ladle the hot soup over them.
Decorate with a few pieces of avocado, cheese and rings of chilli.
Place limes and sour cream on the table, along with the rest of the avocado, cheese and chillies, so that each person may add according to taste.