Cut away the fat from the veal kidneys. Cut the underside of the kidneys and remove the white strands inside. Season the kidneys with pepper. Heat butter in a frying pan and brown the veal kidneys on all sides. Fry in the preheated oven at 220 °C for about 12 minutes.
For the mustard sauce, peel and finely chop the shallots. Heat the butter in a saucepan and sauté the shallots until translucent. Add herbs and spices and deglaze with white wine. Pour in soup and boil down to a sauce. Add cream, crème fraîche and mustard and simmer again. Season to taste with a squeeze of lemon juice and strain the mustard sauce through a hair sieve.
Remove the veal kidneys, wrap in aluminum foil and let rest for another 5 minutes. Cut into slices, sprinkle with fleur du sel and serve with the mustard sauce.