1.Preheat oven to 220°C/425°F.
2.Quarter fennel lengthways; chop and reserve 1 tablespoon fennel tips.
3.Lightly spray heated large shallow baking dish with oil; cook fennel over heat until browned lightly. Transfer to oven; roast fennel, uncovered, 15 minutes.
4.Meanwhile, score fish three times each side. Rub fish all over with rind; fill fish cavity with lemon slices. Roast fish on top of fennel, uncovered, about 20 minutes or until fish is cooked.
5.Serve fish and fennel sprinkled with juice and reserved tips.