Chop and prepare your vegetables. Slice the leeks then place in a colander and rinse with cold water to wash away any dirt that is trapped. Make sure to break up the rings releasing any dirt in between. Peel the sweet potatoes then slice in cubes. Rinse and dry the zucchini then slice in cubes.
In a large dutch oven or stock pot, heat oil over medium, add leeks and garlic. Saute until softened and fragrant, about five minutes. Add the sweet potatoes, zucchini, vegetable broth, salt, pepper and optional turmeric, bring to a boil, reduce heat to low, cover and simmer for 30 minutes.
Let cool for about 15 to 20 minutes. Using a hand held submersion blender, blend until creamy. If you choose to use a blender or food processor to blend your soup, do it in two batches.
Serve hot, chilled or room temperature. The soup will thicken as it cools.
If you find the soup too thick, add a bit of milk, 2 tablespoons at a time until it reaches your preferred consistency.