Heat oil in a large saucepan. Add onion and garlic, and sauté over very low heat for 15 minutes. Add leek, zucchini, potato, and cauliflower.

Add vegetable stock (or boiling water) and simmer for an additional 30 minutes. Add additional two cups of water, salt, and pepper, and continue cooking until vegetables are fully softened.

Cool slightly and blend using an immersion blender. If desired, add heavy cream or soy milk. (Sub for other non-dairy milk alternative if preparing on Passover.)