1/2-3/4cupreserved chicken broth from poaching chicken
salt and pepper
Place chicken breasts in a pan and add just enough water to cover. Add about 1/2 teaspoon salt, 1/4 teaspoon pepper, and 1/4 teaspoon thyme, plus 1/2 onion, 1 celery stalk, and carrot.
Bring to a boil and immediately reduce to a simmer for 2 minutes. Remove pan from heat and cover. Let sit 30 minutes. Remove chicken from liquid and set aside. Save 3/4 cup of broth. Dice chicken once cooled.
Preheat oven to 350 degrees.
Heat olive oil in a nonstick skillet. Add diced onion, celery, and sliced mushrooms. Saute until lightly browned. Season with salt and pepper.
Add chicken soup, mayo, milk, and reserved chicken. Mix well. Stir in 1 cup of stuffing mix.
Spray a casserole dish with cooking spray and transfer chicken mixture to dish.
Add remaining 3 cups of stuffing mix on top. Pour melted butter on top and then chicken broth. Pour chicken broth on slowly and evenly so that the stuffing mix soaks it up as much as possible.