Cook the chicken. Place 1 whole chicken, 2 quartered onions, 4 cloves garlic, 3 dried ancho chiles, 1 tablespoon kosher salt, 1 teaspoon whole black peppercorns, and 1 teaspoon whole cumin seeds in a large stockpot. Add enough water to cover the chicken by about 2 inches.

Simmer. Bring to a boil over medium-high heat, then reduce the heat until the liquid is barely simmering. Cook until the chicken is tender and cooked through, about 1 hour.

Cool the chicken. Remove the chicken from the pot and set aside until cool enough to handle.

Strain the stock. Strain the cooking liquid through a fine-mesh strainer into a large heatproof container and discard the solids. Reserve 1/2 cup chicken stock for the taco filling. Let the remaining stock cool, then refrigerate for up to 1 week or freeze for up to 3 months for another use.

Shred the chicken. Remove and discard the skin and bones, then shred the chicken into large pieces.

Make the pico de gallo. Combine 2 diced ripe mangos, the juice of 1 lime, 1 seeded and diced jalapeño, 1/4 cup chopped cilantro, 1/4 cup diced red onion, and 1/2 teaspoon kosher salt in a medium bowl. Taste and add more lime juice or salt as needed.

Cook the onion. Heat 2 tablespoons oil in a large frying pan over medium heat. Add 1 thinly sliced onion and season to taste with salt. Cook, stirring occasionally, until the onion begins to brown, about 3-5 minutes.

Finish the chicken filling. Add 3 chopped chipotles in adobo, the reserved chicken stock, and the shredded chicken. Cook, stirring occasionally, until everything is well combined and the chicken is heated through, about 3 minutes.

Add the lime. Stir in the juice of 2 limes and season to taste with salt if needed.

Serve. Divide the chicken among 16–20 warmed corn tortillas and top with the mango pico de gallo and any additional toppings like sour cream, avocados, and/or cotija cheese. Serve immediately.