The chicken has to be cooked first. Place it in a large cooking pot with enough water. Add ¼ cup of the sliced onion and the garlic to the pot containing the chicken. Now cook the chicken until it loses its pink raw colour and turns white. Remove the chicken from the broth and keep the cooking liquid. Place the chicken in a mixing bowl and shred the meat from the bones. Keep the shredded meat and discard the bones and the skin.
After the chicken is done, place a skillet on the stove. Heat the oil in it and fry the remaining onions in it. Remove the onions when they turn transparent.
While the onions are being fried, place the tomatoes in a pan. Pour over some of the chicken cooking liquid and boil the tomatoes. The tomatoes are done when they split open.
Pour the boiled tomatoes into a food blender. Top it with the can of chipotle peppers and chicken bouillon. Process the three ingredients well into a smooth puree. Now season with salt and ground pepper (preferably fresh) according to taste. Then, pour this mixture out into a large cooking pan and start heating it. Add the chicken and the fried onions. Stir them together and heat the mixture through.
When the dish is heated through, remove it from the pan and pour it out into a serving dish. Place this dish next to the bowl of sour cream and tostada shells. Serve and enjoy.