Place the potatoes into a pan of boiling water with a pinch of salt and cook for 20-25 minutes until tender, adding the celeriac after 5 minutes. Drain and mash until smooth, then beat in a knob of the butter with the milk. Season with a pinch of salt and pepper and keep warm until needed.
Meanwhile, toss the flour and paprika together in a shallow dish and season with a pinch of salt and plenty of pepper, then use to coat the turkey strips. Heat a knob of the butter and half the oil in a frying pan over a medium heat. Add the turkey and stir-fry for 3-4 minutes, until sealed and lightly browned. Tip onto a plate and set aside.
Reheat the pan over a medium heat. Add the rest of the butter and oil and then add the celery. Sauté for 2–3 minutes, until softened but not coloured. Increase the heat to high and then add the white wine vinegar and allow to bubble right down, then add the stock, wine, tomato purée and mustard, stirring to combine. Tip the sautéed turkey with any juices back into the pan, then bring to the boil. Reduce the heat and simmer gently for about 5 minutes, until the sauce has reduced by half, stirring occasionally.
Stir the soured cream or crème fraiche into the pan and return to the boil, then reduce the heat again and stir in the oyster mushrooms. Simmer gently for a couple of minutes and cook until the sauce has thickened and slightly reduced, and the mushrooms are tender. Divide the celeriac mash warmed plates and spoon over the turkey and oyster mushrooms Stroganoff. Add a sprinkling of paprika to serve.