I adapted this recipe from Barefoot Contessa. She used it on fish and I made it with chicken breasts. It was very tasty and good enough for company.
4 (8-ounce) fish fillets such as red snapper or 4 boneless skinless chicken breasts
Kosher salt and freshly ground black pepper
8 ounces good mayonnaise
3 tablespoons Dijon mustard
1 tablespoon whole-grain mustard
2 tablespoons minced shallots
2 teaspoons drained capers
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.
Combine the mayonnaise, 2 mustards, shallots, capers, 1 teaspoon salt, 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 ( you will need to bake 30-40 minutes for chicken), 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 from the pan spooned over the top.