In a skillet, melt margarine over medium heat; cook zucchini with onion, stirring occasionally, 7 minutes or until very tender; set aside. In blender, sprinkle unflavored gelatin over cold water; let stand 2 minutes. Add hot milk and blend at low speed until gelatin is completely dissolved, about 2 minutes. Add sour cream, cheese, dill, salt, pepper, then zucchini mixture. Blend on high speed until smooth, about a minute. Pour into bowl; chill for about 2 to 4 hours, or until set. Whisk until smooth, then serve with bread sticks, crackers, or assorted vegetables.