Preheat the oven to 200C.
Combine garlic, sage, 1 tablespoon oil and 1 tablespoon caramelised balsamic in a bowl. Add the beef and turn to coat. Place beef at one end of a baking paper-lined baking tray. Spread the cauliflower over the rest of the tray and drizzle with 2 tablespoons oil.
Roast for 15 minutes, then remove tray from oven and sprinkle cauliflower with the parmesan, chilli and remaining 1 tablespoon caramelised balsamic. Return the tray to the oven and roast for a further 12-15 minutes until cauliflower is golden and beef is cooked to medium. Rest beef, loosely covered with foil, for 5 minutes.
Meanwhile, toss the cauliflower with the rocket. In a separate bowl, whisk lemon juice, beef resting juices and remaining 1 tbs oil. Drizzle over the cauliflower salad.
Thinly slice the beef. Divide salad among bowls and top with beef to serve.