In a medium saucepan bring the potatoes, salt and vegetable stock to a boil. Turn down heat and simmer for 4 to 5 minutes, remove from liquid and set aside. In a small bowl, combine the grainy mustard, Dijon mustard, rice wine vinegar, horseradish, lemon juice and salt and pepper. Whisk in oil. In a medium saucepan, bring the milk, thyme, bay leaf and garlic to a boil. Turn heat to low and poach black cod in liquid for 4 to 5 minutes. Remove from heat and keep warm. Blanch baby leeks in water, remove and set aside.
In a small bowl, toss arugula, chickweed, onion and radishes with a tablespoon of vinaigrette, season with salt and pepper.
To serve: Place the room temperature potatoes with baby leeks on 4 plates. Place salad on top of the potatoes and leeks. Place warm smoked black cod on top of salad. Top with small dollop caviar.