Husk and rinse the tomatillos, cutting out the stems. Roast the tomatillos in the oven at 400F until you need them (15 minutes or so). Add two chipotles in adobo and 2 peeled garlic cloves to a blender. I usually de-stem and de-seed the chipotles.
For the chicken, add the seasoning ingredients to a bowl: 1/2 teaspoon chili powder, 1/2 teaspoon Mexican oregano, 1/4 teaspoon salt, freshly cracked black pepper, and a dollop of oil. Chop up the chicken breast into bite-sized pieces and combine well with the seasoning.
Roughly chop 1/2 onion and add most of it to a skillet over medium heat along with a dollop of oil. Once the onion is sizzling add the seasoned chicken pieces and form a single layer with them. Saute until the first side is nearly cooked through (5-8 minutes), then flip and cook the second side for a couple minutes to finish them.
When the chicken goes on I usually add the tortillas to a dry skillet over medium heat along with some slices of cheese. Cook the tortillas until crispy and light brown spots are forming on the bottom.
As the chicken cooks you'll have time to make the Guac. Finely chop 2-3 tablespoons of onion. Use the back of a fork to smoosh the onion until it turns translucent. Add the onion to a bowl along with the flesh of one avocado, 1/4 teaspoon of salt, and the juice of a half lime. Mush and mix well. Taste for seasoning, adding more salt and lime juice if necessary.
Just before serving, add the roasted tomatillos to the blender and combine well with the chipotles and garlic. If it won't combine readily you can add a splash of water to it. Give it a taste and confirm that the Salsa is warm (and that you like the heat level, adding more chipotle if you want). If the Salsa cools down before serving I will usually warm it up in a saucepan.
Each crispy, cheesy tortilla gets chicken, Guac, and plenty of Salsa. Buen Provecho!