1. Preheat oven to 400 degrees.

2. Season chicken with salt and pepper and wrap in parchment-paper-lined foil; place on a rimmed baking sheet. Transfer to oven and bake until chicken is cooked though, about 1 hour.

3. Remove chicken from foil and pour any juices that have accumulated into a medium bowl. Remove skin from chicken and discard. Remove chicken from bones and cut into small pieces; discard bones and set chicken aside.

4. Add salsa, both soups, and onions to bowl of accumulated juices; stir to combine. Spread 1/2 cup of the salsa mixture in bottom of a 9-by-13-inch baking dish. Place half of the tortilla strips in an even layer over salsa mixture, followed by half of the chicken, half of the mushrooms, half of the remaining salsa mixture, half of the sour cream, and half of the cheese. Repeat process with remaining ingredients. Cover and refrigerate overnight.

5. Uncover and transfer baking dish to oven and heat oven to 300 degrees. Bake until casserole is heated through and cheese is melted, about 1 hour. Serve immediately with cilantro, if desired.