Brush one side of 1 slice of bread with the honey. Sprinkle the rosemary and pepper over the honey. Sprinkle the nuts over, then add half the Kerrygold Blarney Castle Cheese, the onion (if used), and the remaining cheese. Spread one side of the second slice of bread with the mustard and place over cheese, mustard-side down.
Brush top of sandwich with 1 teaspoon of the butter. Place buttered side down in a small nonstick skillet over medium-low heat. Spread top with remaining teaspoon butter. Cook 2 to 3 minutes or until the underside is golden brown. With a wide spatula, turn the sandwich over. Place a small heatproof plate on top and cook until the underside is golden brown and the cheese has melted, 2 to 3 minutes more. Alternatively, use the spatula to flatten the sandwich. Serve immediately.
*Other bread options: multi-grain, oatmeal, pumpernickel, rye