Rich salmon stands up to an assertive mustard sauce. The combination of grainy and Dijon mustards gives both body and toothsomeness to the sauce.
Preheat oven to 450 degrees. Place salmon on a rimmed baking sheet; season with salt and pepper. Roast until salmon is opaque throughout, 8 to 10 minutes.
Meanwhile, in a small bowl, stir together the mustards, sugar, and 1 tablespoon water until smooth. Stir in dill and scallion. Serve salmon with sauce.