Preheat oven to 400 degrees F.
Cut the chicken breast into 1-inch cubes.
Warm the olive oil in a 10-inch oven-proof skillet over medium-high heat. Add the chicken in an even layer and sprinkle with salt and garlic powder. Allow to cook for several minutes, then use a fork to flip the pieces over. Yes, use a fork, not a spatula. Just trust me.
Turn the heat down to low after you flip the chicken. Cook for 2 more minutes or until you don’t see any pink in the chicken. Pour in 1/4 cup of tomatillo salsa (don’t wash the 1/4 cup—you’ll need it again!) and, using two forks (this is why I made you use a fork! No need to dirty a spatula, too) tear the chicken into small pieces. If you want to add the spinach, stir it in now and cook until wilted, a couple minutes.
Turn off the heat. Stir in 1 cup of the cheese and all of the sour cream.
Wrap the tortillas in a damp paper towel and microwave in 10 second intervals, until pliable. Don’t skip this step: they’ll rip!
Holding a tortilla in one hand, scoop 1/4 cup of the mixture into each tortilla, roll it up, and place it seam-side down onto a plate while you roll the others.
Pour a couple tablespoons of the salsa verde into the skillet (it’s fine if the skillet still has chicken or sour cream or cheese in it!) and spread it around the bottom to prevent the enchiladas from sticking. Place the tortillas seam-side down into the skillet.
Pour the remaining salsa all over the tortillas. Sprinkle the remaining cheese over top. Bake for 10 minutes.
Garnish with avocado, cilantro, hot sauce, cotija cheese, etc. if desired. Serve immediately.