Melt margarine in a pot over medium heat. Cook and stir onions in melted margarine until translucent, 5 to 7 minutes. Transfer onions to a blender with a slotted spoon, leaving pot on heat.
Cook and stir zucchini in the pot until tender, 7 to 10 minutes. Pour 1 cup chicken stock over the zucchini; stir. Add zucchini and stock mixture to the blender.
Blend onions, zucchini, and stock until smooth; return mixture to the pot. Add remaining chicken stock and milk to the mixture; stir. Season with salt and pepper. Cook until heated through, about 5 minutes.