Heat 1 tbsp of the oil in a heavy-based frying pan and sauté the shallots, garlic, pancetta and thyme together over a moderate heat until the shallots are soft and the pancetta lightly golden. Remove from the pan and set aside
Place another tablespoon of olive oil into the pan with the mushrooms and continue to fry until the mushrooms are just cooked. Combine them with the shallots and pancetta mix. Keep warm.
Season the steak with salt and pepper. Heat the remaining oil. Add the steaks and fry for 2 ½ minutes on each side for rare. Add the butter for the final minute of cooking to help brown the steak and to keep it moist, spooning it over as the beef cooks. Spread some mustard over the top of the steaks. Transfer the beef to a warm plate and leave to rest.
Deglaze the pan with the red wine vinegar and reduce until almost fully evaporated. Add the tomato puree, stock and crème fraiche. Reduce the liquid down by a third until thickened slightly. Return the shallots, pancetta, garlic and mushrooms and their juices to the pan then check for seasoning. Finish with a few knobs of butter for a velvety finish. Add the parsley to the sauce just before serving.