Saute onion in 4 tablespoons melted butter until soft. Add chicken breasts and cover with water. Bring to a boil. Reduce heat and simmer until the chicken is tender (30 - 40 minutes). Add salt, pepper, and sage.
Remove chicken from broth. Cool; then shred.
Preheat oven to 375 degrees.
Melt the one stick butter and pour into a casserole dish. Place chicken on top of butter.
Whisk flour, milk, and spread on top of chicken. Mix 2 cups chicken broth, soup, and sour cream. Whisk. Pour over chicken and flour mixture. Do not stir. Bake 30 -40 minutes until golden brown.
Cool 15 minutes. Pour 1 cup warm chicken broth over top. Let stand 5 minutes.