2.Place beef in a shallow roasting tray. Using your hands, rub butter all over fillet and sprinkle evenly with salt and pepper. Roast 30-35 minutes for rare or 40-45 minutes for medium rare. Remove from oven, tent with foil and stand 20 minutes. Remove string and slice thickly.
3.Meanwhile, to make onion marmalade, combine onions and oil in a large frypan and cook over moderate heat until onions are soft, about 8 minutes, stirring frequently. Season with salt and pepper, then stir in wine, balsamic and sugar. Bring to a simmer, then reduce heat to low and simmer gently, uncovered, for 40 minutes or until liquid has reduced to a syrup, stirring from time to time. Allow to cool in pan, then transfer marmalade to an airtight container. Refrigerate and serve chilled or at room temperature.
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