Clean, wash and cut the soup greens into pieces. Bring 2 litres of water, soup greens, 1 teaspoon of salt, peppercorns and bay leaves to the boil. Wash the meat, add it and let it simmer at low heat for about 1 1/2 hours.
Take the meat out and let it cool down. Pour stock through a sieve and use it for other purposes. Put the egg yolks, 1 pinch of salt, pepper and lemon juice into a narrow tall mixing bowl and mix with the whisks of the hand mixer.
Add oil slowly while stirring continuously. Drain the tuna and puree with 1 teaspoon of capers using the cutting stick of the hand mixer. Stir into the mayonnaise and season again with salt, pepper and lemon juice.
Cut the meat into thin slices, arrange them on a plate and pour the tuna mayonnaise over them. Spread the rest of the capers on top. Garnish with basil. Serve with romaine lettuce with tomatoes and onions.