Preheat oven to 350°F. Layer the blanched string beans on the bottom of a greased casserole dish. Melt butter in a large skillet, sauté the mushrooms, onions and garlic in the butter until the onions are soft. Add the can of cream of mushroom soup and soy sauce, stir to combine. Pour mixture over the string beans and sprinkle with panko bread crumbs. Bake for 20 minutes or until browned.