Whisk the creme fraiche, horseradish, honey and chives and add enough water to make a drizzleable dressing. Season well and chill while you make the beef. Put the onions in a bowl and add a little splash of vinegar and water. Toss and leave to sit.
Heat the oven to fan 180C. Oil the beef all over and season really well. Heat a non-stick frying pan to very hot and add the beef. Sear all over, turning until a good colour on all sides.
Transfer to a small roasting tin and roast for 10 minutes or until medium rare. Remove and cover with foil and leave to cool to room temperature.
To assemble, tear the lettuce and wash and dry the leaves. Put on a platter. Slice the beef thinly as possible and drape all over the lettuce. Scatter the onions and then drizzle with the dressing. Finish with more chives.