The chicken can be anything you like, breasts, thighs, legs and thighs. My personal opinion would be thighs, lots of flavor in thighs. Use bone in and skin on chicken.
Place the chicken in a pot and cover with water. Cover and cook on medium heat for 1 hour or until the chicken is cooked through. Remove from heat and let this cool to room temperature, this ensures juicy chicken.
Once the pot has cooled to room temperature, preheat your oven to 180 C (350 F).
Remove the chicken from the pot. Strain the liquid and reserve, there is your broth to use.
Remove the skin and bones, and any cartilage and shred the chicken use large forks. If making the soup from a shortcut, reserve about 2-3 tablespoons of chicken and dice that up and set aside, use that for the soup shortcut. Place the shredded chicken in a 7x11 baking dish, spread the chicken out to make a even layer.
Whisk together the flour, milk, and butter, pour over the chicken. Do not stir.
Whisk together the broth, soup, salt and pepper, pour over the casserole, Do not stir.
Bake for 45-50 minutes or until the top starts to brown up a bit. All ovens are different, so you may have to bake longer or shorter. Remove from the oven and let it rest for 5-10 minutes before serving. Enjoy.