Heat oil in a large pot over medium-high heat. Add onion, leek, and garlic, and cook for 3 - 4 minutes until they are translucent.

Add zucchini and sauté for 5 – 6 minutes, tossing to coat in the oil. Add broth and water. Bring to a boil, then cover and reduce heat to medium.

Cook for 15 - 20 minutes or until zucchini is very soft.

Using an immersion blender, puree the soup in the pot.(Alternatively, you can puree it in a food processor and return it to the pot.) Add the yogurt, milk, and lemon zest. Add salt and pepper to taste. Bring back to a simmer, whisking to distribute the yogurt and milk evenly. Remove from heat.

Serve in bowls, swirl in a teaspoon of yogurt or two if desired, and a little more pepper. Serve hot. Alternatively, you can also chill the soup and serve it cold.