Melt the butter and the oil in a large saucepan and brown the pheasant breasts. Put on a plate.
Add the onion, garlic and bacon and cook for 3-4 minutes until the onion starts to soften.
Stir in the flour and cook for a minute. Add the cider, stock , vinegar, thyme and bay leaf. Season well. Bring to the boil and replace the pheasant pieces. Turn the heat down so the liquid bubbles gently and cover. Cook for 20-30 minutes until the pheasant is cooked and the sauce has thickened. Don't overcook or the meat will dry out.
Meanwhile bring a large pan of salted water to the boil and add the potato, apple and celeriac cubes. Boil until tender, drain, add the parsley and sour cream and some ground black pepper. Mash until smooth. Serve with the pheasant casserole and a green salad.