Preheat the oven to 220C/425F/Gas 7 (fan 200C).
Rub the fillet of beef with plenty of salt and pepper and the oil.
Heat a large frying pan until very hot. Fry the fillet on all sides until browned. Transfer to a roasting tin.
Place the fillet in the centre of the roasting tin, arrange the whole garlic bulbs around the fillet and drizzle with a little oil. Roast for 23 minutes for medium-rare (only 20 minutes if it is a long, thin piece).
When the beef is cooked, remove from the oven, cover with aluminium foil and leave to rest for 15 minutes.
Serve hot or you can leave it to cool, and then wrap in more foil and place in the fridge to cool completely.
To make the sauce, cut the roasted garlic bulbs in half horizontally and squeeze the garlic paste from each single clove into a bowl. Add the mayonnaise, crème fraîche and sugar.
Blend to a smooth purée using a blender or food processor. Stir in the wholegrain mustard. Chill until ready to serve alongside the beef.
Carve the beef into thin slices just before serving.