Step 1

On medium heat, melt the butter with the oil in a large saucepan or dutch oven.

Step 2

Add the onion and cook until soft.

Step 3

Stir in the leeks and potatoes and cook, turning occasionally, until the potatoes are evenly browned.

Step 4

Add salt and pepper, then the stock and milk. Bring to the boil, stirring constantly.

Step 5

Reduce heat to low, cover and simmer for 20 - 25 minutes, or until the potatoes are very tender.

Step 6

Puree the soup in a blender, or alternatively, pour the soup through a strainer into a bowl, using the back of a spoon to rub the vegetables through. Discard any remaining pulp.

Step 7

Return soup to the saucepan and simmer for 5 minutes. Serve at once.