Butter a skillet or a shallow baking dish and arrange cod on it. Sprinkle with salt and pepper. Mix sour cream or creme fraiche, both mustards, chopped leek and capers in a medium bowl. Spoon the sauce over the fish, it should be covered in an even layer of sauce.
Roast in the oven for 10-12 minutes. Change your oven setting to broil/grill and continue cooking for 2-3 more minutes until the sauce is slightly blistered. Sprinkle with fresh parsley and serve.