Wash the tuna, dab dry and sprinkle with lemon juice. Heat 2 tablespoons of oil in a pan. Pat the tuna steaks dry and fry on both sides over medium heat for 4-5 minutes.
Season with salt and pepper. Wash parsley, dab dry, pluck leaves from the stalks and chop finely. Mix with lemon peel and sea salt. Clean and wash the salad and cut the larger leaves into bite-sized pieces.
Clean, wash and slice the tomatoes. Peel onion and cut into rings. Line a large plate with lettuce, tomatoes and onion rings and arrange the tuna steaks on top. Sprinkle with the parsley and lemon mixture and sprinkle everything with the remaining olive oil.
Garnish with lemon wedges as desired and serve.