Adapted from this Food.com recipe.
2 Tbsp coarse-grain mustard
2 Tbsp balsamic vinegar
1/2 tsp kosher salt
1/4 tsp fresh-cracked black pepper
2 fillets of salmon
Combine the mustard, vinegar, salt, and pepper. Poor mixture over the salmon and let marinade in a shallow dish or sealable plastic bag for 30 minutes. After marinading, place the fish skin-side up on a well-oiled grill heated to 350-375 degrees, saving the remaining marinade. After four minutes, flip the fish over and baste the cooked side with left over sauce. Remove from grill when cooked through, about four additional minutes.