Preheat oven to 220°C. Arrange lemon halves, cut-side down, on a baking tray and drizzle with 50ml oil. Season. Roast for 35 minutes or until soft and juicy. Cool slightly, then using a lemon squeezer, squeeze out juice. Discard peel or roughly chop and add to dressing (depending how lemony you like it). Whisk in garlic and remaining 100ml oil, and season. Transfer to a jar and chill for up to 1 week. Serve with salad greens or roasted broccoli, or spoon over cooked prawns or fish.