Preheat the oven to 200C/400F/Gas 6.
For the steak, heat a griddle pan until smoking. Rub the steak with the oil then season both sides with salt and freshly ground black pepper. Add the steak to the hot griddle and chargrill for 3-4 minutes on each side, or until cooked to your liking. Remove form the heat to rest for 2-3 minutes.
To make the sauce, melt the butter with the oil in a frying pan over a medium heat. Add the onions and garlic and cook until softened.
Add the mushroom and sauté until cooked through.
Add the red wine and beef stock and bring to the boil. Reduce the heat and simmer until the liquid has reduced and thickened, then add the chervil and stir.
To serve, place the steak in the middle of a warm plate and spoon the sauce around.