3 tablespoons extra-virgin olive oil
2 1/2 pounds trimmed, skinless, bone-in chicken thighs and breasts
1 Tablespoon whole peppercorns
1 Bay Leaf
Salt and freshly ground black pepper
1 large onion , thinly sliced
3 large garlic cloves , minced
One 28-ounce can diced tomatoes
2 canned chipotles in adobo, coarsely chopped
24 corn tostada shells
2 ounces Cotija cheese, crumbled
Sliced scallions, avocado and chopped cilantro, for garnish
Season the chicken with salt and pepper. Place in a large stock pot with six cups of water, peppercorns and bay leaf. Bring to a boil over high heat, and then reduce to a simmer over medium-low heat for 20 minutes. Turn the heat off and remove the chicken from the liquid. Reserve the broth for later use. Using 2 forks or clean hands, shred the chicken into a bowl and set aside
Add 3 tablespoons of olive oil to a medium skillet along with the onion. Cook over moderately high heat, stirring occasionally, until the onion is lightly browned and softened, 5 minutes. Add the garlic and cook until fragrant, 2 minutes. Add the tomatoes and their juices, 1 cup of the reserved stock, the chipotles and bring to a boil. Simmer over moderate heat, stirring occasionally, until thickened and slightly reduced, 20 minutes.
Transfer the sauce to a food processor and let cool for 15 minutes. Puree until smooth and season with salt and pepper. Pour the sauce over the chicken.
Spoon about 3 tablespoons of chicken onto each tortilla and sprinkle with the crumbled cheese. Garnish the chicken tacos with the scallions, sliced avocado and chopped cilantro.