Preheat oven to 350 degrees.
Lightly butter a large shallow baking pan.
In a small bowl, combine the mayonnaise and mustard.
Pat the fillets dry with paper towels.
Arrange fish in prepared pan.
Spread mayonnaise mixture over each fillet.
Sprinkle each with the pecans; press down gently.
Bake 12 to 15 minutes or until fish flakes easily with fork.