Line a sheet pan with parchment paper or foil. (You can also use an ovenproof baking dish.) Place the fish fillets skin side down on the sheet pan. Sprinkle generously with salt and pepper.
Combine the crème fraiche, two mustards, shallots, capers, 1 teaspoon salt, and ½ teaspoon pepper in a small bowl. Spoon the sauce evenly over the fish fillets, making sure all the fish is covered. Bake for 10 to 15 minutes, depending on the thickness of the fish, until it’s barely done. (The fish will flake easily at the thickest part when it’s done.) Be sure not to overcook it! Serve hot or at room temperature with the sauce in the pan spooned over the top.