For the salad, mix sour cream, vinegar and 2 tablespoons of sugar. Season with salt and pepper. Peel and wash the celery and cut into fine strips with a slicer. Wash the apple and slice the pulp around the core into fine strips. Mix sour cream, celery and apple.
Preheat oven (electric cooker: 150 °C/circulating air: 130 °C/gas: see manufacturer). Wash herbs and shake dry. Dab meat dry and season with salt. Heat the oil in a roasting pan and fry the meat in it for approx. 8 minutes. After about 6 minutes add herbs. Deglaze with sherry and white wine. Continue cooking in the hot oven for about 15 minutes.
Wash, pluck and halve the grapes. Clean, wash and halve the romaine lettuce lengthwise and cut into fine strips. Fold the grapes and romaine lettuce into the apple-celery mixture. Season salad to taste. Coarsely chop the walnuts and sprinkle over the salad.
Remove meat from the roaster and keep warm. Pour the meat through a sieve into a pot and bring to the boil. Mix flour and 2 tablespoons of water and pour into the boiling frying set while stirring. Bring to the boil and simmer for about 5 minutes. Refine with cream and season to taste with salt, pepper and sugar. Cut meat into slices. Arrange salad, meat and sauce.