Mash the avocado with the lime juice, 1/4 teaspoon red pepper flakes, 1/4 teaspoon salt, and 1/4 teaspoon black pepper in a small bowl.
Using tongs, toast each tortilla directly on the grate of a gas burner turned to medium-high. Cook, flipping once, until the edges are charred and the tortilla is hot. If you don't have a gas burner, microwave the tortillas until hot, 15-30 seconds.
Spread the mashed avocado mixture on the toasted tortillas, dividing it equally. Top with the chicken, salsa, cheese, and pickled onions. Sprinkle with cilantro leaves, additional red pepper flakes, and additional salt (if desired). Serve with lime wedges alongside.
Two Healthy Kitchens' notes from our testing of this recipe:
Chicken: You can gently rewarm the chicken before making your tacos, but it's delicious even if you don't bother to do that.
Salsa and cheese: We loved this recipe's flavors made with salsa verde and crumbled cotija cheese and, as noted in our post, we felt like those both added uniqueness to these tacos. But, feel free to mix and match salsas and cheeses, or even offer a variety of choices to create a fun taco bar.
Make-ahead steps: You can have pretty much everything prepped ahead of time for this recipe, so it can be very quickly assembled and served on busy evenings. The lime juice in the avocado mash helps to keep the avocado from oxidizing and turning brown, so you can make the avocado mash a day or so ahead and stash it in the fridge. To be extra-sure your avocado doesn't brown, press plastic wrap directly on the surface of the avocado mixture before refrigerating. You can also shred the chicken ahead of time and make the Quick-Pickled Red Onions ahead (up to two weeks in advance). With everything prepped ahead, this meal takes just minutes to finalize!