Season the flour with salt and pepper on a plate. Cut the pork cheeks into chunks and coat with the flour.
Melt half the butter with the oil in a flameproof casserole and fry the pork in batches until browned on all sides, lifting out with a slotted spoon on to a plate. Melt the remaining butter in the casserole and fry the mushrooms for 5 minutes, until all their juices have evaporated and they are pale golden. Lift out on to a separate plate and set aside.
Return the pork to the casserole with any flour left over from coating and cook, stirring, for 1 minute. Stir in the onions, then blend in the cider and stock. Bring to the boil, stirring, then cover and cook in a preheated oven, 150°C (300°F), Gas Mark 2, for 2½ hours or until the meat is very tender, adding the tarragon and mustard halfway through cooking.
Return the mushrooms to the casserole and stir in the cream. Heat through gently, season to taste with salt and pepper and serve.