COOK zucchini in a small amount of water in a large saucepan for about 10 minutes or until tender. Drain.
PLACE half zucchini, half evaporated milk, half water and half chicken stock in bowl of food processor or blender container; process until smooth. Place in large bowl. Repeat with remaining halves. Add to bowl and set aside.
MELT butter in large saucepan on medium heat. Add onion; cook until soft, about 3 minutes. Stir in zucchini mixture. Season with salt and pepper; cover. Bring to a boil (about 15 minutes), stirring occasionally.