2 tablespoons Dijon mustard
2 teaspoons white wine vinegar
2 teaspoons brown sugar
1/2 teaspoon salt
1 1/2 to 1 3/4 pounds salmon, cut into approximately 6-ounce portions
In a small bowl, whisk together the mustard, vinegar, brown sugar and salt. Spread the mixture over all sides of each piece of salmon. Cover and marinate in the refrigerator for 15 minutes.
Meanwhile, place a cast iron skillet or other heavy oven-safe pan a few inches below the broiler and preheat for 10 minutes.
Carefully remove the hot skillet from the oven and place the salmon skin-side up in the pan. (Depending on the size of your broiler and pan, you may need to cook the fish in batches.) Broil for 2 minutes. Flip and broil for another 1 to 2 minutes, depending on the thickness of the fillets, until the salmon is just barely translucent at its thickest point. (It will continue cooking off the heat.) Remove from the hot pan immediately.
Serve hot or refrigerate it to serve it cold the next day.