Preheat oven to 350 degrees. Slice zucchini lengthwise and parboil 5 minutes. Scoop out center, leaving approximately ¼ inch in the shell; reserve pulp. Melt butter in pan and saute onion. Stir in pulp, shrimp, and remaining ingredients. Stuff shells with shrimp mixture and place on ungreased cookie sheet. Bake 30 minutes.