For the duck, preheat the oven to 230C/450F/Gas 8.
Squeeze the Seville oranges, reserve the juice and stuff the ducks with the skins. Truss the ducks and season the skin with salt and pepper. Place on a rack sat in a roasting tray.
Roast the ducks in the hot oven for 45 minutes to an hour - they should be slightly underdone. Pour off the fat from the roasting pan. Pour the orange liqueur over the ducks and set alight to flame the duck. Place them on a dish, breast-side down, and keep in a warm place.
Drain any remaining fat from the roasting pan, then pour in the wine, deglaze the pan by scraping off the juices from the bottom. Heat and reduce in volume to a thick sauce. Add the orange juice, heat again to reduce in volume by half, then beat in the butter and check the seasoning.
For the savoy cabbage with smoked bacon, discard the tough dark green outer cabbage leaves. Stack the leaves in threes and remove the large middle veins with a sharp knife. Chop coarsely.
Heat half the oil in a large frying pan. Put in the carrots and cook until tender, then stir in the garlic and cook for a few minutes, stirring continuously. Transfer the vegetables to a bowl, using a slotted spoon.
Fry the bacon in the same pan until brown and crisp, then lift it out with the slotted spoon and put with the carrot.
Cook the celeriac in the bacon fat until lightly brown and tender and put with the carrots and bacon.
Add the remaining vegetable oil to the pan. When hot, cook the cabbage, season with salt and stir well so the cabbage does not burn.
Cover with a tight fitting lid and cook over a high heat, shaking the pan briskly. The heat will make the cabbage shrink and loose volume.
Cook until tender, then mix in the bacon and the other vegetables. Check the seasoning.
To serve, carve the ducks, arrange them on a serving dish with the orange segments and coat with the sauce. Pile the cabbage alongside.