Pound chicken breasts flat. Saute spinach in olive oil and garlic powder for 4 minute (I use fresh chopped garlic) Spread spinach equally over chicken breasts. Sprinkle 1/4 cup feta cheese over each. Roll chicken breasts closed and seal firmly with toothpicks. Slightly brown chicken breasts on all sides in butter. Place breasts in ungreased baking dish with tight lid and bake covered for 1 hour at 350 degrees. (do not add liquid; will make its own).
Saute mushrooms in butter. Add flour, stirring until well coated. Add chicken broth from baking dish and simmer until thickens. Pour over chicken breasts to serve.