Crush mustard seeds and juniper berries. Wash the fillet of beef. Put mustard seeds, bay leaf, juniper berries, red wine and beef fillet in a freezer bag and close it tightly. Marinate beef fillet overnight, at least 12 hours
Quarter the pumpernickel and put it on a baking tray. Dry in the preheated oven (electric cooker: 150 °C/ circulating air: 125 °C/ gas: stage 1) for approx. 15 minutes. Take out and let cool down
Remove the beef fillet from the marinade, dab dry and season all around with salt and pepper. Heat the oil in a pan and fry the meat all around. Place the meat and roasting set on a baking tray and cook in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 25-30 minutes
Peel, wash and cut the celery into pieces. Peel, wash and chop the potatoes. Cook celery and potatoes in boiling salted water for about 15 minutes
Take the meat out of the oven, cover with aluminium foil and let it rest for about 10 minutes
Peel garlic and chop very finely. Wash the orange and grate the peel thinly. Wash parsley, shake dry and chop the leaves finely. Crumble the pumpernickel in the universal chopper. Mix orange peel, pumpernickel, parsley, olive oil and garlic and season with salt and pepper.
Drain the celery and potatoes. Add butter and cream and mash. Season to taste with salt and nutmeg. Cut meat into slices. Arrange meat and puree on plates and sprinkle with the Gremolata. Finally sprinkle with meat juice
Waiting time approx. 12 hours