Clean and slice the leeks, using only the white and light green parts. Rinse thoroughly to remove dirt.

In a large pot, heat the butter or olive oil over medium heat. Add the leeks and sauté until soft, about 7–10 minutes. Add garlic and cook for 1 minute more.

Stir in the diced potatoes and pour in the vegetable broth. Bring to a boil, then reduce heat and simmer until potatoes are soft, about 20–25 minutes.

Use an immersion blender to puree the soup until smooth. Or, carefully blend in batches using a blender.

Stir in the cream (if using), and season with salt and pepper. Warm through and serve hot with herbs or crusty bread.