1. For the vinaigrette: In a bowl, add the chopped anchovies, lemon juice, olive oil, dijon mustard, salt and pepper and chives and mix together to emulsify.
2. Season the tuna steak with the blackened seasoning. Grill this along with the lettuce head.
3. Cook for 2-3 minutes on each side.
4. Plate the grilled tuna steak along with the lettuce head and tomatoes and onion. Pour the vinaigrette over. Garnish with parmesan cheese and serve.