Preheat oven to 170 degrees C. Melt 150g butter in a saucepan and add orange and lemon juices. Brush the turkey inside and outside with this mixture (reserve the rest for basting) and season with salt and pepper.
Heat the olive oil in a saucepan and saute the onion until softened. Add the beef and pork mince and cook until the meat is browned. Stir in the rice, then add the chestnuts, 75g butter, crumbled chicken stock cube, water and Cognac. Season with salt and pepper to taste and simmer until the rice has absorbed the liquid. Stuff the turkey with this mixture and tie with kitchen string.
Place the turkey on a rack in a roasting pan and cover loosely with foil. Pour 5mm water into the bottom of the pan (maintain this water level as cooking). Cook in the preheated oven 3-4 hours, basting regularly with the orange-lemon mixture and topping up the water as needed.
Increase the oven temperature to 200 degrees C for the last 1/2 hour of cooking and remove the foil. The turkey should be at 80 degrees C internal temperature and golden. Let stand 20 minutes before carving.