Put the shallots, carrot and celery into a large saucepan with the bay leaf, thyme, peppercorns, wine and chicken stock. Add the flaked sea salt and bring to a gentle simmer. Cook the stock for 30 minutes.
Using a sharp knife, trim the veal of any fatty bits or sinew. Lower the veal gently into the warm stock and reduce the heat until it is barely bubbling. The veal won’t quite be covered by the stock, but it will be turned during the cooking process.
Poach the veal for 15 minutes, turning 4-5 times. Remove from the liquid, put onto a board and set aside to cool. Reserve 100ml/3½fl oz of the cooking liquor.
Once the veal is cool, season it all over with freshly ground black pepper and wrap tightly in cling film. Put in the fridge and leave overnight to chill.
To make the mayonnaise, put the egg yolks, tuna, capers, one tablespoon of the lemon juice, mustard and sugar in a food processor. Season with salt and a couple of twists of freshly ground black pepper.
Blend until well combined then, with the motor running, very gradually add both the oils and blend until the dressing has emulsified to a mayonnaise consistency and is smooth and thick.
Add two tablespoons of the reserved cooking liquid and blend for a few seconds more until the sauce has a soft spoonable consistency, adding a little more cooking liquor if necessary. You may need to remove the lid and push the mixture down a couple of times with a rubber spatula until the right consistency is reached.
Place the tuna mayonnaise into a bowl, season with more salt and pepper and the remaining squeeze of lemon juice if you like. Cover the surface with cling film to prevent a skin forming and chill overnight.
When ready to serve, unwrap the veal and slice very thinly. Arrange overlapping slices of the meat on six plates and top with spoonfuls of the chilled mayonnaise. (Or make one large platter.)
Sprinkle with baby capers and scatter roughly chopped parsley over the top. Garnish with caper berries, sprinkle with coarsely ground black pepper and a pinch of salt. Serve with extra lemon wedges for squeezing. Serve with very chilled white wine.