Combine the sour cream, heavy cream, 2 mustards, shallots, capers, 1 teaspoon salt, dill and 1/2 teaspoon pepper in a small bowl. Spoon the sauce evenly over the fish fillets, making sure the fish is completely 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.)
Use any type of fish including red snapper, halibut, salmon, etc.
Preheat the oven to 425 degrees F.
Line a sheet pan with parchment paper. (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. Top with the sauce and bake until done. (about 10 minutes per inch of thickness)