Grind fennel and coriander seeds with a pestle and mortar.
Rub the sugar and orange zest together until fragrant, then add the ground spices, sage, tarragon, mustard, and olive oil to make a paste.
Spatchcock the chicken and generously spread the marinade all over. Set aside on a wire rack on a tray.
Preheat the oven to 220°C.
Oil a baking pan, add orange slices, fennel, and shallots, and add orange juice and a pinch of salt.
Place the chicken on top and pierce the skin all over with a knife.
Loosely cover with foil and bake for 20 minutes. Remove the foil and bake for 30-40 minutes, basting every 10 minutes.
Carve and serve.