Layer the blanched string beans in the bottom of a greased casserole dish. Melt butter in a large skillet. Saute the mushrooms, onion and garlic in the butter until the onions are soft. Add the Cream of Mushroom soup. Stir to combine soup and vegetables in the skillet, pour mixture over the string beans. Sprinkle with Panko Bread Crumbs. Bake at 350 degrees F for 20 minutes or until breadcrumbs are browned.