To make the pie, heat 1 tbsp of the butter in a casserole dish or shallow saucepan. Season and fry the guinea fowl legs for 10 mins until browned. Transfer to a plate and set aside, then add the bacon to the pan and fry for a few mins until just starting to crisp. Scatter in the onions, carrots and bay leaf and cook for 5 mins more until softened, adding a bit more butter if needed. Stir in the flour and wine, if using, and boil for 30 seconds. Gradually pour in the stock, stirring constantly, and return to the boil.
Return the legs to the pan, season generously, reduce the heat and simmer gently for 50 mins until the meat is tender and falling off the bone. If the stock doesn’t cover the legs completely, turn them over halfway through cooking. Transfer the legs to a plate and leave to cool, then stir in the cavolo nero, simmer for 1 min more and turn off the heat. When the legs are cool enough to handle, shred the meat off the bone and stir it back through the sauce. Tip everything into a small pie dish.
Boil the potatoes in salted water for 10 mins until tender, then drain and mash with the milk, most of the remaining butter and some seasoning. Top the pie with the mash and set aside. Will keep in the fridge for up to two days.
Heat the oven to 220C/200C fan/gas 7. Heat some oil in an ovenproof frying pan and brown the crown, skin-side down. Turn using tongs, add the clementines to the pan, cut-side down, and put the thyme in the cavity of the crown. Transfer the pan to the oven. Dot the pie with the remaining butter and put in the oven on the shelf below the crown. Roast the crown for 25 mins, or until it reaches 60C on a cooking thermometer, then remove from the oven and leave to rest for 10 mins while the pie continues to bake until golden. Serve the pie on a board with the crown and the caramelised clementine halves for squeezing over.