Preparation

1. Preheat oven to 375 F. Warm olive oil in a large skillet or everyday pan over medium high heat. Add sliced mushrooms and cook, stirring often until they begin to brown. Add about 3/4 of the diced onion and continue to cook, stirring often for another 4-5 minutes or until the onions have softened. Stir in about 1/2 cup of the tomatillo salsa. Add the chicken then the spinach in batches and stir until the spinach is wilted (it is a lot of spinach - but it shrinks). Season with salt and pepper as desired. Place the remaining diced onion in a bowl of ice water. Dice the tomatoes and place into a bowl with the cilantro.
2. In an 8" x 8" baking dish, spread about 1/3 cup of the tomatillo salsa on the bottom of the dish. Cut 3 of the tortillas in half, then cut half of the halves into 2 pieces. Pour the tomatillo salsa into a shallow bowl or pie plate. Dip one whole tortilla into the tomatillo salsa, then place into the baking dish. Dip the half tortilla into the salsa and place next to the whole tortilla then the 2 quarter pieces of tortilla into the salsa and place on the bottom of the baking dish to cover the bottom in an even layer. Pour half of the chicken mixture into the baking dish over the tortillas. Sprinkle about 1/3 of the shredded cheese over the chicken mixture. Repeat with the next layer of tortillas, remaining chicken mixture and 1/3 of the cheese. Top with the last layer of tortillas, spreading any remaining tomatillo salsa over the top before sprinkling with remaining cheese. Cover with aluminum foil and bake for 30 minutes.
3. Strain the onions from the ice water and blot with towel to remove excess water. Add onions to the tomatoes and cilantro, stir to combine. Season with salt, pepper and maybe a pinch of sugar if desired.
4. After 30 minutes, remove from oven, uncover carefully and check the temperature to make sure it is greater than 150 F. Set aside to rest for about 10-20 minutes to allow the dish to settle and relax before cutting. Serve with fresh tomato/onion salsa. Also great with beans and rice - here's a link to one of our favorite recipes: https://hr.umich.edu/benefits-wellness/health/mhealthy/physical-well-being/nutrition/mhealthy-recipes/one-pot-beans-rice