Cut potatoes in half lengthwise and cook in little boiling salted water for about 20 minutes. In the meantime wash tuna steak, pat dry and sprinkle with 2 tablespoons of lemon juice. Season with pepper and set aside. Cut onion into fine rings. Wash the rosemary, dab dry and chop coarsely, except for a little to garnish. Wash parsley, dab dry and chop as well.
Wash lettuce and tomatoes and drain well. Quarter the tomatoes and pluck the lettuce into bite-sized pieces. Mix vinegar, salt, pepper and mustard. Add oil and fold under. Mix the tomatoes with the vinaigrette and arrange on a plate with the salad. Heat the fat in a pan. Fry the steak for 4-5 minutes while turning. Finally add onion rings and rosemary and fry. Season with salt, remove from the pan and leave to rest for 4-5 minutes wrapped in foil. Dissolve the gravy with 2 tablespoons of water and the remaining lemon juice. Stir in pesto and parsley. Drain the potatoes.
Fry the steak for 4-5 minutes while turning. Finally add onion rings and rosemary and fry. Season with salt, remove from the pan and leave to rest for 4-5 minutes wrapped in foil. Dissolve the gravy with 2 tablespoons of water and the remaining lemon juice. Stir in pesto and parsley. Drain the potatoes. Arrange steak with potatoes and warm pesto sauce. Serve garnished with remaining rosemary