Heat the olive oil in a medium saucepan over medium heat. When the oil shimmers, add the chicken, onion, and a pinch of salt and pepper. Cook stirring the onion and turning the chicken occasionally, until the chicken is browned all over and the onion is fragrant, 3 to 5 minutes per side. Stir in the enchilada sauce and the chipolte peppers. cover, reduce the heat to low, and simmer until the chicken has cooked through and tender, about 15 minutes. Remove the pan from heat and, using two forks, shred the chicken directly in the pan.
To serve, spread the mashed avocado over the tortillas., dividing evenly, and add the chicken on top. Top with salsa (if desired), cheese and cilantro.
Pickled Jalapeño Pineapple Salsa
In a medium bowl, stir together the pineapple, jalapeno, lime juice, cilantro, and a pinch of salt. Store the salsa refrigerated in an airtight container for up to 2 days.