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Up to 2 days before, crush together the peppercorns and seeds using a pestle and mortar, or in a saucepan with the end of a rolling pin or similar. Brush the beef fillet all over with the mustard, then roll in the peppercorn mix to coat. Cover and chill.
Two hours before you are ready to start cooking, take the beef out of the fridge to come to room temperature.
Heat oven to 200C/180C fan/gas 6. Heat the oil in a pan big enough to hold the beef, and brown the beef all over, seasoning with salt. In a roasting tin, use the onions as a bed to sit the beef on – pat on any spices that have fallen off. Roast for 20 mins for rare, 25 mins for medium, and 35 mins for well-done beef.
Lift the beef from the tin and rest on a platter, covered with foil to keep it hot, for about 30 mins. Set the tin with the onions and roasting juices over the hob and stir in the oil, cornflour and crumbled stock cubes. Mix in the jelly, then gradually stir in the wine and 400ml boiling water, scraping up any stuck bits from the bottom. Simmer for 15 mins until reduced a little. Season, then sieve into a clean pan to keep warm, discarding the onions. Slice the beef, scatter over the thyme, if you like, and serve with the gravy.