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The day before; put all the marinade ingredients into a non-metallic bowl; mix well. Swish in the steak slices to coat them well. Cover and leave for at least two hours or overnight.
To serve; preheat oven to 200C/Gas 6/fan oven 180C. Lift the steaks out of the marinade and pat dry with kitchen paper. Strain the marinade and onion, reserving both.
Bake the loaves following the pack instructions. Heat a frying pan to searing hot, season and fry the steak in batches for 1-2 minutes on each side. Keep warm.
Heat 1 tablespoon of the olive oil in the pan. Tip in the onion and fry for 1-2 minutes. Pour in the marinade, add a little salt and bubble to reduce slightly.
Halve the loaves horizontally. Drizzle the cut sides with the remaining olive oil. For each loaf, fill with four radicchio leaves topped with four steak slices, some onion and juices. Cut each loaf into four and serve with your favourite mustard.