Heat a griddle pan until smoking. Place the steak medallions on the pan and cook for 1 minute on each side, or until cooked to your liking. Remove from the pan, set aside and let rest in a warm place for five minutes.
For the sauce, place the Guinness in a small pan and bring to the boil. Let it reduce by ¾.
Add the sugar and vinegar and stir well.
Serve the steak with the sauce poured over it.
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