Into a medium-size pot, add chicken breasts, the halved onion, garlic cloves, bay leaves, black peppercorns, and 2 teaspoons of salt. Cover with water by about ½” and bring to a boil. Reduce heat to a simmer, cover, and cook until the internal temperature reaches 165º at the thickest part of the breast, about 10 to 15 minutes. Transfer breasts to a cutting board.
Add the tomatoes to the simmering cooking liquid. Cook for 2 to 3 minutes, or until the tomatoes have slightly softened and the skin begins to peel away. Transfer the tomatoes to a plate or bowl to cool, then peel them.Drain the cooking liquid, reserving ½ cup of liquid. Discard peppercorns and bay leaves, but reserve onion halves and garlic cloves. Into a blender, add the peeled tomatoes. cooked onion halves, garlic cloves, chipotles, adobo sauce, cumin, oregano, and ½ cup of the cooking liquid. Blend until smooth, then season with salt and pepper to taste.
In a large skillet over medium high heat, heat oil until it shimmers. Add sliced onion and cook until slightly softened and beginning to take on some color, 4 to 5 minutes.
Meanwhile, shred the chicken breasts.
Into the skillet, carefully pour in the tomato mixture. (There may be some splattering, but the more you stir the less there will be.) Bring the sauce to boil, then reduce to a simmer. Cook until the sauce reduces a bit and thickens slightly, about 5 minutes.
Add the shredded chicken and stir, breaking up the bigger shreds. Cook for an additional 3 to 4 minutes, or until the chicken has heated through. Occasionally stir to coat chicken in the sauce.
Spoon chicken onto corn tortillas and top with minced onion, cilantro and avocado. Serve with lime wedges.