In a large pot, boil chicken breasts with 1 onion in approximately 8 cups of water until tender.

Remove the skin and bones, then shred or cut the chicken into bite-sized pieces.

Reserve the chicken stock.

Preheat the oven to 350°F (175°C).

Spray a 13 x 9 inch baking pan with cooking spray.

Tear small pieces of bread and layer them in the bottom of the prepared pan.

Slice the remaining onion thinly and lay it on top of the bread.

Sprinkle diced celery and poultry seasoning over the onions.

Arrange the cooked chicken pieces on top of the celery and onions.

Melt the butter and drizzle it over the chicken pieces.

In a bowl, combine the cream of mushroom soup and cream of chicken soup.

Pour the mixture evenly over the chicken.

Arrange sliced sharp cheese over the top of the casserole.

Bake in the preheated oven for about 45-50 minutes, or until the cheese is melted and bubbly, and the casserole is heated through.

Serve hot and enjoy!