Heat the oven to 180C/160C fan/gas 4. Toss the sweet potatoes with a generous drizzle of oil and some seasoning, and tip into a very large roasting tin. Push the potatoes to one end of the tin, then, in the other end, toss the chicken with the onions, spice mix, a drizzle of oil and some seasoning. Roast for 40 mins, stirring everything halfway through. Add the broccoli to the tin, drizzle with a little oil and season, then roast for 10-15 mins more.
Remove the chicken, onions and broccoli from the tin. Roughly mash the potatoes using a fork, making sure you incorporate all the chicken juices and spices from the pan. Spread the mash over the base of the tin, then top with the broccoli, chicken and onions and serve from the tin in the middle of the table.