Stir mustard powder with mustard seeds, 3 tbsp cold water, vinegar and salt in a small bowl. Stir 2 tbsp chopped fresh dill into mustard mixture. Store in a glass jar with a tight-fitting lid. Refrigerate overnight before using.
Brush 4 salmon fillets with 2 tbsp Dill Mustard on a parchment-lined baking sheet. Season with 1/4 tsp salt. Bake in centre of a 425F oven until a knife tip inserted into the thickest part of salmon and held for 10 sec comes out warm, 10 to 12 min.