Tie the beef eye of round roast tightly with kitchen twine if it has not already been tied by the butcher.

Cut the leek and onion into large pieces.

Place the beef eye of round roast in a large pot with 600 ml water and 400 ml dry white wine.

Add the leek, onion, 2 to 3 black peppercorns, and parsley sprigs to the pot.

Bring the poaching liquid to a gentle boil over the stovetop.

Cook the beef at a gentle boil for 75 min.

Turn off the heat and cool the beef completely in its cooking broth.

Remove the cooled beef from the broth and wrap it tightly in plastic wrap.

Refrigerate the wrapped beef overnight, or for at least 8 hours, until very cold and firm.

Place the canned tuna in water, large egg, 200 ml extra virgin olive oil, 1 tsp capers, whole-grain mustard, anchovy fillets, lemon juice, and salt in a tall blending container.

Rest the immersion blender on the bottom of the container and blend without moving it until the sauce begins to emulsify.

Raise the immersion blender slowly until the tonnata sauce is smooth and thick.

Slice the chilled beef as thinly as possible with a fine slicing knife.

Arrange the thin beef slices in overlapping layers on a serving platter.

Cover the beef slices generously with the tonnata sauce.

Sprinkle the dish with the remaining capers and chopped chives.

Serve the remaining tonnata sauce separately in a serving bowl.