One to two days before, place the meat, onion, carrots, celery, garlic, bouquet garni, juniper berries and salt and pepper in a large glass or earthenware bowl. Pour in enough red wine to just cover the meat, then add the vinegar. Cover with plastic wrap, then refrigerate, the longer the better.

Remove meat from marinade and pat dry. Heat 2 tablespoons of butter and the oil in a large Dutch oven over high heat until hot but not smoking. Reduce heat to medium, add the meat, and brown on all sides, about 15 minutes. Off the heat, flambé the roast with the Cognac. (To flambé: heat alcohol in saucepan, then remove from burner. Have lid at the ready, just in case. Stand back, and ignite liquid with long match. Pour flaming alcohol over meat.).

Add the marinade, and all the ingredients in it. Simmer, partially covered, over low heat until the meat is tender, about one hour. Transfer meat to a cutting board, and let rest for 10 minutes before slicing.

Strain the cooking liquids through a sieve, reserving the vegetables. Discard garni. Return the the liquid to the Dutch oven, and boil, uncovered, over high heat to reduce by one third, about 5 to 7 minutes.

Finish the sauce: Purée the vegetables and cooking liquid in a blender to a smooth consistency. It should be a thick, full-flavored sauce. Return to the pan and reheat it. Add the red currant jelly and whisk until well blended. If the sauce seems thin, add a little potato starch dissolved in 1 tablespoon water or wine. Whisk in the remaining 2 tablespoons of butter. Do not boil.

Slice the meat, and arrange on a platter. Spoon some of the sauce over the sliced meat, and pass the rest in a sauceboat.