Preheat BBQ grill to a medium heat.
Grease two squares of aluminum foil with butter, and evenly divide fish into two portions.
In a small saucepan, melt butter, add lemon juice, parsley, dill weed, salt and pepper. Mix well. Pour equal amounts over the two portions of fish.
Sprinkle with paprika and top with sliced onion and finely grated Parmesan or Asiago cheese.
Fold the foil securely around the fish, leaving a small space for the fish to expand.
Grill for 5-7 minutes each side. Test to ensure fish flakes in centre to determine doneness.