Preheat the oven to 200C/390F/Gas 6.
Heat a frying pan until medium hot, then add the duck skin-side down and fry for 2-3 minutes to render out the fat from under the skin. Drain the fat off carefully, discard, then return the pan to the heat with the duck.
Add the star anise, orange zest, orange juice and half of the port to the pan. Bring to a simmer, turn the duck over (so that it is skin-side up) to coat in the liquid, then place into the oven for 12-15 minutes, or until the duck is golden-brown and cooked but still slightly pink in the middle. Transfer the duck to a warm plate to rest.
Return the pan to the heat and bring to a simmer, then stir in the remaining port and cook for 1-2 minutes. Whisk in 50g/2oz of the butter until melted, then season with salt and freshly ground black pepper. Keep warm over a very low heat.
Heat a frying pan until hot, add the remaining butter, winter greens and a splash of water. Cook for 3-4 minutes, or until the greens are tender. Season with salt and freshly ground black pepper. Keep warm.
For the cranberry chutney, place the water, sugar and spices into a pan and bring to the boil. Add the fresh and dried cranberries and the port and return to the boil, cooking for a further 3-4 minutes, mashing the cranberries down as they soften into a thick, slightly sticky consistency.
For the celeriac mash, place the celeriac and potatoes into a pan of salted water, bring to the boil and simmer for 12-15 minutes, or until tender. Drain, return to the pan and heat to drive off any excess moisture.
Transfer the celeriac and potato to a food processor and blend to a smooth purée. Add the olive oil and butter, season to taste with salt and freshly ground black pepper and pulse once more to combine.
To serve, pile the winter greens onto the centre of each serving plate. Spoon some of the celeriac and potato purée alongside. Thickly slice the duck and lay over the top of the greens, spoon the chutney alongside and drizzle the sauce over.
