Heat the olive oil in a medium-sized saucepan over medium heat until it begins to shimmer. Add the chicken, onion, a pinch of salt and a pinch of pepper (if you don't actually pinch, use about ⅛ tsp). Cook, stirring occasionally and turning the chicken over, until chicken is browned all over and the onion is fragrant, about 3-5 minutes on each side. Add the enchilada sauce and chipotle peppers to the pan. Cover with a tight-fitting lid, reduce the heat to low, and simmer about 15 minutes, until the chicken is fully cooked and fork-tender to shred. Remove the pan from the heat; using two forks, shred the chicken in the pan, coating shredded pieces with sauce.
To serve, spread the mashed avocado over the tortillas, top with chicken, crumbled cheese, and salsa (if desired... highly recommended!)
In a medium bowl, combine the pineapple, jalapeño, lime juice, cilantro, and salt, and gently toss together. Use the salsa right away or refrigerate it in an airtight container for up to 2 days.
Source: Half Baked Harvest Super Simple