Season the veal fillet with salt and pepper and fry briefly on each side in clarified butter in a hot pan. After searing, place the veal fillet on an oven rack and cook pink in the 160 degree oven for about 20-25 minutes. Then leave to cool on a wire rack.
For the glazed onions, fry the halves in oil in a small pan until golden brown. Then season with salt and pepper and pour in the veal stock until the onions are almost covered. Braise the onions in the pan in the oven at 200 degrees for about 45 minutes.
For the tuna sauce, sauté shallots and mushrooms in butter. Add tomatoes, garlic, celery, carrots, capers and anchovy fillet and sauté briefly. Drain the tuna, mix in and deglaze with Nolly Prat and white wine. Pour in the tomato stock and season with bay leaf, thyme, rosemary, salt and pepper. Stir in the grated potato and simmer until the vegetables are soft. Puree the sauce in the mixer, strain through a sieve and reduce to approx. 300 ml. Then stir quickly cold on crushed ice. Stir in mayonnaise and sour cream and season to taste with lemon juice and cayenne pepper.
Cut the tuna and the saddle of veal into very thin (approx. 1mm) slices, preferably with a slicer, and arrange on plates together with the glazed onion halves. Spread the tuna sauce over it and garnish everything with a few wild herb salad leaves.