Chop any green leafy fennel fronds and set aside. Cut off the stalk-like fennel top, remove the outer layers and finely chop both. Heat 1 tbsp butter in a frying pan and cook the onion, garlic and chopped fennel until soft but not coloured.
Add the rice and stir for 1 min. Pour over most of the wine and simmer until evaporated. Add 500ml of the hot stock, a ladleful at a time, stirring between each addition until it is absorbed.
Meanwhile, slice remaining fennel and fry in knob of butter until browned. Add remaining stock and wine and cook until tender.
When the rice is cooked, stir in the zest, Parmesan and some seasoning. Take off the heat and set aside, covered, for 2 mins. Serve in bowls, topped with the fennel fronds and cooked fennel, stirring the prawns through if using.