Step 1Preheat oven to 180º. Line a large baking dish or tray with baking paper. Mix vinegar with mustard in a small dish.
Step 2Spray a large non-stick frying pan with olive oil and set over high heat. Brown the beef for 1–2 minutes each side, or until golden. Transfer beef to prepared tray; brush beef on all sides with mustard mixture. Add beetroot and carrots to tray, lightly spray with oil. Roast vegies and beef for 40 minutes, for medium, or until beef is cooked to your liking.
Step 3Transfer beef to a plate, cover loosely with foil and rest for 10 minutes. Return vegetables to oven for a further 10 minutes, or until golden and tender.
Step 4Meanwhile, cook cauliflower and potato in a saucepan of boiling water for 10–12 minutes, or until tender. Drain, return to pan, add horseradish, then mash until smooth. Stir through chives.
Step 5Slice roasted beef and serve on cauliflower mash with roasted beetroot and carrots.