To make the stock, chop the carcass bones into several pieces and roast in a hot oven for 10 minutes at to 200°C/Fan 180°C/Gas 6.
Once browned, put the bones in a large pan with the chopped onion, celery, carrot, garlic and leek. Cover with cold water and bring to the boil.
Simmer for 1 hour 30 minutes and keep topping up the water so that everything is just covered.
Pass the stock through a fine sieve into another pan, then add the diced potato, onion, garlic, leek, mushrooms, and thyme. Gently cook until soft, then add the stock and cream and blend it all together.
Remove the skin from the breasts and gently poach them in the stock for 20 minutes. When cooked, carefully take from the pan and put to one side.
Stir the cream into the stock, then carefully ladle it into a food blender. Blitz until smooth and velvety, then return to the pan and bring back to a simmer.
Chop the pheasant into roughly ½ cm pieces and add to the soup. Season to taste and pour into soup bowls. Sprinkle the fresh chopped parsley over the top and serve with warm sourdough chunks and proper salted butter.