In a medium saucepan, bring potatoes, salt and stock to a boil. Turn down heat; simmer for 4 to 5 minutes.
When cooked, remove potatoes from liquid and allow them to cool.
In a small bowl, combine the grainy mustard, Dijon mustard, rice wine vinegar, horseradish and lemon juice.
Whisk in oil until emulsified – you can also use a hand-held blender.
In a medium saucepan, bring the milk, thyme, bay leaf and garlic just to a boil.
Turn heat to low and poach fish in liquid for 4 to 5 minutes. Remove from heat and keep warm.
In a small bowl, toss the baby greens, onion and radishes with two tablespoons of the vinaigrette and season with salt and pepper.
The remaining vinaigrette may be stored, refrigerated, in a sealed jar
Place the rose potatoes and baby leeks on four plates. Place dressed salad greens on top of the potatoes and leeks.
Place warm sablefish on top of salad. If fish is cold, reheat in poaching liquid. For an elegant touch, garnish the dish with caviar on top of the cod.