In a large pot, sauté leeks and garlic in oil, until fragrant, about 8 minutes.
Add potatoes and broth, and bring to a simmer for 30 minutes.
In batches, very briefly process in blender (no more than half full) with milk.
Return soup to pot and heat until hot.
Season with salt and pepper to taste and serve with chives or scallions, if using.
