Remove all skin and dark red areas from the tuna. Cut or chop the tuna into small pieces (similar to minced meat, but not too fine), mix with the lemon juice and marinate for about ½ hour.
Finely chop the onion and onion greens or chives. Using a fork, finely mash the anchovies and capers together with the onions and onion greens and mix with the tuna together with the olive oil. Season to taste with salt and pepper.
Serve with toasted white bread slices.
Note: Recipes for tuna tartare are available worldwide. It is seasoned with local ingredients, e.g. in France with tarragon mustard.