To a small pot, add the chicken, onion cut in half, whole garlic clove, sofrito and one teaspoon of salt. Let boil until the meat has cooked and become tender, about 20 minutes.
Remove chicken from the pot and shred the meat using two forks.
In a blender, mix the onion and garlic in the broth with the chipotle peppers, the tomato that has been cut in half, one teaspoons of salt and the left over chicken broth.
In a medium skillet, heat up a the oil over medium-high heat. Sautee the chopped onion until it has become transparent. Add the minced garlic, chopped tomato, shredded chicken and chipotle mixture.
Bring up to a boil, and let simmer until the broth has reduced almost completely, about 15-20 minutes.
To assemble the tostadas
Warm up the tostadas in the oven at 350 degrees for about 5 minutes.
Peel and roughly mash the avocado. Place about two tablespoons of avocado at the bottom of the tostadas. Add about 2-3 tablespoons of chipotle chicken, then cheese and a tablespoon of pico de gallo salsa. Finally, top with sour cream.