Melt the butter in a heavy-bottomed saucepan. Add the onions and mushrooms and gently fry until soft.
Add the sherry to the pan and cook quickly, stirring.
Pour in the stock and cook over a medium to high heat until reduced by half.
Meanwhile, heat a griddle pan or frying pan until hot. Lightly brush each steak with the oil and place on griddle. Cook each side on a medium heat until done as preferred.
Place steaks on serving place and spoon the sauce over them.
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