Preheat the grill and set the oven rack about 15cm from the heat source.
Place the fillets on the grill pan and squeeze the lemon over the top of the fish. Grill the fillets in the preheated oven for about 6 to 8 minutes.
Combine parmesan, butter, Dijion, mayonnaise, garlic and salt in a small bowl. Remove the fish from the oven and spread the cheese mixture on top of the fillets. Sprinkle with paprika. Grill until the topping is lightly browned and the fish flakes easily with a fork, about 3 minutes.