Preheat oven to its highest setting. Toss broccolini in 1½ tbsp oil, then arrange in a roasting pan in a single layer, season to taste and roast until starting to brown (8-10 minutes).
Heat remaining oil in a frying pan over low heat, add garlic and fry, stirring occasionally, until dark golden (1-2 minutes). Remove with a slotted spoon and set aside. Add chilli to pan, cook until fragrant (1 minute), then remove from heat and add vinegar (take care, mixture will spit). Add roast broccolini to pan, season to taste and serve hot, scattered with crisp garlic.