Brush the beef with the horseradish. Heat 1 tablespoon of the oil in a frying pan over a high heat, add the beef and sear on all sides until browned. Transfer to a roasting tin and place in a preheated oven, 200°C (400° F), Gas Mark 6, for 20 minutes. Cover with foil and leave to rest for 5–6 minutes, then slice.
Meanwhile, cook the quinoa in a saucepan of boiling water for 8–9 minutes, or according to the pack instructions.
Heat the remaining oil in a large frying pan, add the onion and cook for 2 minutes, then add the mushrooms and cook for a further 5–6 minutes until softened. Remove from the heat and stir in the chopped herbs.
Drain the quinoa and stir into the mushroom mixture. Divide between 4 warm plates and top with the rocket and sliced beef. Serve drizzled with the balsamic glaze.