Crush the peppercorns and bay leaves roughly in a mortar and pestle. Add the salt, crushing it slightly, but don't grind finely. Rub the mixture all over the duck breasts. Place them in a dish, cover with clingfilm and refrigerate for at least two hours (overnight if possible).
Remove the cabbage's softer outer leaves and hard central core. Halve and shred the cabbage.
Heat the butter in a heavy-bottomed casserole and gently sauté the onion until it softens – it shouldn't brown. Add the cabbage and spice and turn everything over in the butter, gently cooking for about two minutes.
Add the sugar and half the cider vinegar (you may not need the rest). Bring to the boil, season, then turn the heat right down, cover the pan and cook very gently for one to 1 hour and 30 minutes, turning the cabbage over every so often.
About three minutes before the end add the jelly, turn up the heat and remove the lid to reduce the cooking juices and melt the jelly – it may take a little longer. The cabbage should start to look more glossy. Taste for the sweet-sour balance – you may need to add the rest of the vinegar.
Keeping the heat medium to high, turn the cabbage with a wooden spoon and allow any juices to evaporate. The cabbage should be quite shiny but not watery. Check the seasoning.
Meanwhile, put the vinegar, sugar and juniper berries in a pan and bring to the boil, stirring to help the sugar dissolve. Simmer for 10 minutes.
Add the onion and simmer for a further 10 minutes. Add the rhubarb and simmer very gently for about four minutes, or until it is just tender. The rhubarb should remain pink and hold its shape. Leave to cool. Cover and refrigerate (but let it come to room temperature before serving).
Preheat the oven to 220C/210C fan/gas mark 7. Brush the bay leaves off the duck. Heat a sauté pan and brown the duck breasts in their own fat, skin-side-down first, for about three minutes. Be careful not to burn the fat.
Quickly brown the other sides, then put the duck breasts, skin-side up, in a roasting-tin and cook for five minutes. (Slice through the centre of the meat to check it's done – it should be pink, like rare steak.) Remove the duck, cover with foil and leave to rest for about four minutes.
Serve sliced on top of the cabbage with the rhubarb alongside.