Light a grill or preheat a grill pan and oil the grate or pan. Brush the salmon fillets with oil and season with salt and pepper. Grill the salmon fillets skinned side down over moderate heat for about 3 minutes. Turn and grill about 3 minutes longer, until the salmon is nearly cooked through. Generously brush both sides of the salmon with the honey mustard and grill, turning frequently, until richly glazed, about 1 minute. (For salmon steaks, grill over moderately high heat, turning once, for 4 to 5 minutes, then brush with the honey mustard, turning frequently, for 1 minute.) Transfer to a platter or plates and serve, passing the remaining honey mustard at the table.