Begin by putting the veal in a medium roasting tin.
Stud the onion with the cloves and add this to the tin, along with the bay leaves, celery, carrot, peppercorns and wine. Then, roast the veal in the pre-heated oven for 1¼ hours. For the sauce, you need to start by making a mayonnaise. So, break the whole eggs straight into the goblet of a blender or food processor, add the garlic clove and 1 teaspoon salt.
Then measure the oil into a jug and switch the machine on. To blend everything thoroughly, pour the oil in a thin, very steady trickle with the motor running. You must be very careful here – too much oil in too soon means the sauce will curdle.
When all the oil is in, add the white wine vinegar and blend. Then add the tuna, anchovy fillets and capers, and whiz again till smooth. Now do a bit of tasting and season with lemon juice and pepper. The sauce can be made well ahead and kept in the fridge till needed. When the veal is ready, take it out of the oven and leave everything to get cold.
After that, take the veal out of the tin – you can discard the vegetables, bay leaves, peppercorns and any remaining wine now. Slice the meat very thinly and arrange it in a large, shallow serving dish. Spoon the sauce over the meat. Now arrange the anchovy slices in a zig-zag pattern on top.
Scatter over a few extra capers and garnish with the lemon slices.