Preheat the oven to 425F. Spread the halved Brussels sprouts out on a parchment-lined baking sheet, drizzle with oil and toss with your hands to coat. Roast for 20-30 minutes, stirring once or twice, until deep golden.
Meanwhile, in a small bowl whisk together the sugar, fish sauce, water, soy sauce, lime juice, garlic and chili flakes.
When the Brussels sprouts come out of the oven, toss them with the cilantro and then a drizzle of vinaigrette (as much as you like of each) while they're still warm. The leftover vinaigrette will keep in the fridge for a few weeks.
Makes as much as you like.