Peel and cut Yellow Onion (1) into quarter inch slices, and set aside.
Finely chop Garlic (1 clove) and set aside.
Cut (score) a two inch x, centered, on the bottom end of the Roma Tomato (1).
Juice Lime (1) into a small bowl.
Remove the stems from the Fresh Cilantro (0.25 ounce) leaves and discard. Roughly chop the leaves, and set aside.
In a medium pot over high heat add Water (8 cup), Chicken Breast (2), garlic, a quarter of the chopped onion, and bring to a boil.
Cook the chicken for eight minutes until fully cooked through and no longer pink.
Remove chicken, cool, and shred meat. Keep water on high heat for the next step. Set shredded chicken aside for plating.
Meanwhile, in a small saucepan over low heat, add Canned Black Beans (15 ounce), lime juice, Ground Cumin (1/2 teaspoon), Salt (1/4 teaspoon), and Ground Black Pepper (1/4 teaspoon). Cook for nine minutes until heated through.
Add half of the cilantro leaves and keep warm for plating.
In the same pot used for the chicken, add the tomato to the boiling water and cook for thirty seconds, then remove.
Carefully peel the skin from the tomato, and discard the skin.
In the base of a food processor, add the peeled tomato, Chipotle Chili Puree (2 tablespoon), Chicken Base (1 package), and Salt (1/4 teaspoon) and Ground Black Pepper (1/4 teaspoon). Process until smooth and set aside.
In a medium saucepan over medium high heat, add Vegetable Oil (2 cup) and heat to 375 degrees F (180 degrees C).
Meanwhile, in a medium saute pan over medium high heat, add Vegetable Oil (1 tablespoon). Add the remaining onions and saute.
Add the shredded chicken and the chipotle sauce. Cook for three minutes until completely heated through.
Add the White Corn Tortilla (4) one at a time into the saucepan with hot oil, and cook on each side for one minute until crispy. Repeat the process until all the tortillas are fried.
Place the fried tortillas on a paper towel to drain.
Divide the tostada shells between two plates. Spread Sour Cream (4 ounce) over the tostada shells and top each shell with chicken tinga mixture and remaining cilantro. Place the black beans in a small bowl on the side.