Trim 1 leek and slice in half lengthwise, then rinse carefully to remove silt. Cut the halves in 2 inch pieces, then thinly slice them lengthwise.
Heat 3 T. olive oil and gently saute the leeks with salt and pepper until they are soft and begin to brown. Turn off the heat and allow to cool, then transfer the oil and leeks into a small bowl.
When the oil is cool, add 2 T. red wine vinegar and 1 t. stone ground mustard plus salt and pepper to taste.
Chop 1/2 C. brined, pitted green olives.
Wash 3 large handfulls of arugula leaves.
Bring a large pot of water to a boil and drop in 1 lb. scrubbed or peeled potatoes.
Cook for 20 minutes, or until you can easily pierce them with a fork. Remove one potato and test to make sure it is done. It should be tender but not crunchy. Drain potatoes and rinse in cool water briefly. Cut into bite-sized dice.
Toss the potatoes with the arugula and olives plus 1/2 t. each salt and black pepper. Add the dressing and toss to coat.