Prepare the Cod: Preheat your oven to 375°F (190°C). Pat the cod fillets dry with paper towels and season both sides lightly with salt and pepper. Arrange the fillets in a lightly greased baking dish, ensuring they are not overcrowded.

Make the Mustard Sauce: In a medium bowl, mix the sour cream (or crème fraîche), Dijon mustard, whole-grain mustard, minced leek, capers, and chopped parsley. Stir until smooth and well combined.

Coat the Cod: Pour the mustard sauce evenly over the cod fillets, ensuring they are well coated.

Bake the Cod: Place the baking dish in the oven and bake for 20-25 minutes, or until the cod flakes easily with a fork and reaches an internal temperature of 145°F (63°C).

Serve: Remove the cod from the oven and let it rest for a few minutes. Garnish with additional parsley if desired. Serve hot with your choice of sides, such as steamed vegetables, mashed potatoes, or a fresh green salad.