Preheat the oven to 220°C (425°F). Line a deep roasting tray with parchment paper and do the same with a smaller tray for the sweet potatoes.

In a pestle and mortar, combine the ingredients for the spice rub and crush them together to release the flavors.

Place the chicken breast-side down on a cutting board. Using kitchen shears, cut along both sides of the backbone.

Turn the chicken over and press down firmly on the breastbone to flatten it. Transfer to the prepared roasting tray.

Rub the chicken generously with the spice rub mixture, saving a small portion for the potatoes. Arrange the garlic and lemon halves around the chicken and drizzle with olive oil.

Place the sweet potato wedges into a plastic bag. Add 1 tablespoon of the spice rub and 1 tablespoon of olive oil, then shake to coat the potatoes evenly. Transfer to the smaller roasting tray.

Position the chicken on the lower shelf of the oven and the sweet potatoes on the upper shelf, or side by side if space allows. Roast for 40 minutes, basting the chicken with the pan juices halfway through. The chicken is cooked when its internal temperature reaches 75°C (165°F), which can be verified with a meat thermometer.

Remove from the oven and allow the chicken to rest for 5-10 minutes before carving.