Transfer the carrots, sweet potato and red onion to the prepared baking tray, drizzle with olive oil and toss until evenly coated. Sprinkle with the chilli flakes, and season with salt and pepper. Roast for 25-30 mins, tossing halfway through, or until tender and turning golden.
NOTE: careful not to crowd the carrots and sweet potatoes, otherwise they will steam instead of roast. If needed, roast on 2 baking sheets, changing positions halfway through roasting time.