Chop garlic finely. Beat 1 tbsp. each lemon and lime juice with 3 tbsp. olive oil and stir in garlic. Season tuna steaks with coarse pepper, put them in a shallow dish, pour the whipped oil over them and let them marinate in the refrigerator for 2 hours.
Take tuna steaks out of the marinade, dry the oil a tiny bit, season with sea salt. Cook in a grill pan for about 3 min on each side, arrange and drizzle with the remaining marinade.
Mix 3 tbsp olive oil with mustard, salt, sherry vinegar and pepper and dress the washed lettuce.
Serve the lettuce with the tuna steak.