Celery and potatoes peel, wash and roughly dice. Cook with lemon juice in boiling salted water for about 20 minutes until soft.
For the sauce, peel the shallot and cut into thin slices. Heat 1 tablespoon of butter. Sauté the shallot in it. Add red wine, bring to the boil and reduce by half.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Wash duck breasts, dab dry, scratch skin crosswise. Season well with salt and pepper. Place the skin side in a cold ovenproof pan.
Turn the heat up fully, fry for about 6 minutes. Turn and fry on the meat side for 2-3 minutes. Place on the skin side again, cook in the oven for about 6 minutes. In the meantime add crème de cassis and currants to the red wine sauce.
Reduce again (5-10 minutes) to half syrupy.
Drain the celery and potatoes. Mash with 100 g butter. Season to taste with salt and pepper. Take duck breasts out of the oven. Let it rest for at least 2 minutes. Remove sauce from the stove. Gradually add 50 g of cold butter to the sauce.
Possibly add some duck fat to the sauce. Serve everything.