Variations: Substitute fillet with sirloin or rump roast.
Pre-heat the oven to 200°C/390°F.
Trim away the silver skin on the fillet then rub the beef with dijon mustard, crushed peppercorns, rosemary and season well with salt.
Heat a large frying pan over a high heat, add a splash of olive oil and evenly brown the fillet on all sides. Sear the ends for only a few seconds to seal in the juices.
Transfer the beef and chopped onion to an oven safe dish and place the garlic cloves on a baking tray and roast in the oven for approximately 20 minutes for medium rare beef. Cooking time will depend on the thickness of the fillet.
Make the horseradish cream by adding the crème fraîche, creamed horseradish and lemon juice to a bowl. Stir well to combine and refrigerate until needed.
Continue roasting the garlic if it’s not soft yet and transfer the fillet to a board to rest for half its cooking time whilst you make the butter gravy.
Pour the beef drippings and onions from the roast’s dish into a small saucepan, pour in the beef stock and bring to the boil and leave to reduce by half.
Remove the sauce from the heat, strain and discard the solids. Whisk in the butter a little at a time until the gravy is glossy. Set aside for serving.
Finish off the horseradish cream by pressing the roast garlic into the bowl with the other ingredients. Stir until well combined and adjust the salt seasoning to taste.
Pour the decadent butter gravy over the fillet and serve with dollops of garlic horseradish cream and roast potatoes.