Remove the outer leaves of the leeks.
As a root vegetable Leeks get very dirty so cleaning them is important. Gently separate the leaves and rung the water through them checking through the various layers for more dirt.
Slice leeks about 1 inch in diameter
Peel and cut potatoes into 2 inch cubes
Heat an 8 quart stockpot on medium heat, add oil and butter
Sautee Leaks 5-7 minutes until they tenderize, add salt and pepper a little bit at a time during each step. Add the Potatoes. Sautee together until well seasoned
Add Chicken or vegetable Broth. Cook on medium low heat until potatoes are soft enough to puree
Puree the mixture until smooth with an immersion blender (or transfer to blender or food processor, but this is more labor intensive and messier)
Serve with Garlic Crostini