Mix butter with fennel seed, garlic, zest, thyme, sugar, salt + pepper.
Spread most under the skin, rub the rest on top.
Stuff the cavity with fennel, citrus, garlic + thyme. Tie with butchers string, tucking in the wings.
In a roasting tray, pour in the EVOO, then layer oranges, fennel + shallots. Pour over the mandarin juice, then season with a good pinch of salt.
Place chicken on top, roast 70–85 mins until thigh reaches 75 °C. Baste once or twice, covering sections with foil if they brown too quickly.
Rest breast-side down for 15 mins. Serve over the roasted base.
