Put chicken into 2 quarts of water. Add sliced onion, garlic, 1 thinly sliced lime and the juice from another, the mint, epazote, bay leaves, oregano and chicken bouillon. Cover and simmer for 40 minutes. Remove chicken and shred. Saute onion, tomato and chiles for 5 minutes. Remove from heat and add lime juice and avocado. Saute tortilla squares in oil until crisp. To serve. Put tortilla pieces in bottom of the soup bowl, add shredded chicken, then broth. Serve tomato-onion mixture on the side as a topping.