Preheat the oven to 200°C.
Season the beef with salt and pepper and sear in a hot pan with olive oil until browned.
Roast the beef for 1 hour 10 minutes for rare.
Cover the beef with foil and rest in a warm place for 15-30 minutes.
Season the flour in a bowl with salt and pepper.
Whisk in eggs and 600ml of milk to create a smooth batter.
Mix in the grated horseradish and chill the batter for at least one hour or overnight.
Preheat the oven to 220°C and melt dripping in muffin tins until smoking hot.
Pour batter into the tins until two-thirds full.
Bake for 20 minutes without opening the oven door.
Reduce heat to 190°C and bake until golden-brown and crisp.
Preheat the grill to high.
Sauté leeks in butter and oil for 1-2 minutes.
Add wine and reduce by half.
Stir in the cream, season, and transfer to an ovenproof dish.
Top with cheese and grill until golden.
Reheat the pan juices from the beef roasting dish.
Carve the beef and serve with the gratin, puddings, and juices.