Wash potatoes thoroughly and cook in boiling water for about 20 minutes. Then drain, peel and slice the potatoes. Clean and wash spring onions and cut them into fine rings.
Clean, wash and halve the tomatoes. Heat olive oil in a pan. Sauté 3/4 of the spring onions in it, stir in mustard and deglaze with lime juice and vinegar. Season with salt, pepper and sugar.
Mix the potatoes, leek marinade and tomatoes and leave to soak. Wash the fish, dab dry with kitchen paper and sprinkle with lemon juice. Heat oil in a pan, fry fish on each side for 3-4 minutes at medium heat.
Then season with salt and lemon pepper. Arrange potato salad and one tuna fillet each on a plate. Garnish with chervil and lime slices as desired.