Pat the spatchcocked chicken thoroughly dry with paper towels. Place it skin-side up on a large baking sheet or roasting pan.
In a small bowl mix together the sugar, salt, sesame seeds and orange zest. Add the ground fennel and coriander seeds and stir with a spoon until well combined. Add the mustard and olive oil and stir to create a paste.
Rub the marinade generously all over the chicken, including underneath the skin where possible.
Place the marinated chicken in the refrigerator uncovered for at least 4 hours, preferably overnight. The cool, dry air helps remove excess moisture from the skin, which encourages beautifully crisp skin during roasting.
When ready to roast, remove the chicken from the refrigerator and let it sit at room temperature for about 20–30 minutes.
Preheat the oven to 400°F (200°C).
Meanwhile, Arrange the potatoes and sweet potatoes in a roasting pan, toss them with olive oil, salt, cumin, and black pepper. Slice the orange into 1cm in thick slices. Tuck the orange slices, garlic bulb, and the onion quarters among the vegetables. Pour the orange juice and olive oil over the vegetables. Place the chicken on top of the vegetables, leaving the chicken skin-side up. Cover the roasting dish with parchment paper and foil.
Roast the chicken and vegetables for approximately 30 minutes, remove the cover and then roast fora noter 30 to 45 minutes depending on the size of the chicken. Rotate the pan halfway through cooking, baste the chicken with the pan juices and toss the vegetables if needed for even browning.
The chicken is done when the juices run clean from thickest part.
Remove the chicken from the oven and let it rest for 10–15 minutes before carving. This allows the juices to redistribute and keeps the meat tender.
Serve the chicken surrounded by the roasted potatoes and sweet potatoes. Garnish with lemon wedges and some dill or thyme.