1.Preheat oven to 240°C/475°F.
2.Rub beef all over with half the oil. Cook beef in a heated flameproof baking dish over high heat on the stove top until browned all over. Spread mustard all over beef; season. Transfer to oven; roast, uncovered, about 15 minutes or until beef is cooked as desired. Remove beef from dish; cover to keep warm.
3.Meanwhile trim beetroot; cut in half (or quarters if large), then wrap in foil; enclose to form a parcel. Roast beetroot in baking dish with beef for the last 10 minutes of beef cooking time.
4.Place kumara, carrots, onion and capsicum on baking-paper-lined large oven tray; transfer beetroot, still in foil, to tray with vegetables. Brush vegetables with remaining oil; season. Roast vegetables about 15 minutes or until tender and beginning to brown. Remove vegetables from tray as they are cooked.
5.Add asparagus to oven tray for last 5 minutes of cooking time or until just tender.
6.Make mustard horseradish dressing. Whisk horseradish cream, garlic, mustard and vinegar in a medium bowl until combined. Gradually whisk in oil then cream. Season to taste. Peel beetroot.
7.Place spinach on a serving platter; top with warm roast vegetables, drizzle with a little dressing. Slice beef thickly; arrange on top of vegetables; drizzle with dressing.
Dressing can be made a day ahead. If your oven or oven tray is small, place the capsicum in the baking dish with the beef. The vegetables should be spread out in a single layer so they cook quickly.Note