First, start to make the stuffed onions. Heat oven to 140C/120C fan/gas 1. Lay the whole onions on a large sheet of foil and wrap up to make a parcel. Cook for 1 hr 30 mins or until soft, then remove and leave to cool. Turn up the oven to 180C/160 fan/gas 4.
While the onions are roasting, heat a medium flameproof casserole dish and add 1 tbsp oil. When the dish is hot, add the pheasant crown and brown on the skin side, then set aside. Add the remaining oil and the onion and fry for 10 mins until soft and starting to brown. Add the garlic and parsnip, cook for a few mins, then add the pearl barley and cider. Simmer until the cider has reduced by half, then pour in the chicken stock and keep on a low simmer for 15-20 mins.
Remove and discard the core from the baked onions using a teaspoon, making them hollow but still keeping them intact. In a mixing bowl, mix all the stuffing ingredients and season. Spoon the stuffing into the hollow onions, making sure that they are full. Roll any extra stuffing into small balls.
Add the pheasant crown and any balls to the simmering barley and sprinkle with a little salt, then add the stuffed onions and thyme. Cover with a lid and put in the oven for 20 mins.
To make the salsa, bring a pan of salted water to the boil and blanch the sprout tops until they are just soft, then plunge them into a bowl of iced water. Drain the leaves and squeeze out any excess water, then chop roughly. Add them to the small bowl of a food processor, along with the mace, chestnuts, garlic, rapeseed oil and lemon zest, and blitz until it becomes a rough salsa texture. Add a little seasoning to taste.
Once the pheasant is cooked, remove the dish from the oven and transfer the pheasant to a plate to rest. Cover loosely with foil. Return the barley to the heat and stir in the mushrooms, apple and parsley – using the mixture to baste the stuffed onions. When it becomes a rich and reasonably thick stew, remove from the heat.
Remove the breasts from the crown with a sharp knife and trim a little to neaten them. Spoon the pearl barley onto a large serving bowl or plate, put the pheasant breast on top and serve with the braised onions and extra meatballs. To finish the dish, add a large spoonful of the salsa and sprinkle over the crispy sage leaves.