Place the fennel in a steamer basket and steam for 5–6 minutes to soften. Refresh under cold running water, drain and pat dry.
Heat 1 tablespoon of the oil and the butter in a heavy-based frying pan, add the fennel and cook for 3–4 minutes on each side until golden brown. Toss in 1–2 teaspoons of the turmeric and set aside.
Heat the remaining oil in the base of a tagine or a large, heavy-based frying pan over a medium heat, stir in the onion, garlic and ginger and cook for 1–2 minutes until beginning to colour. Add the prawns and cook for 2–3 minutes until they turn pink, then stir in the remaining turmeric and the paprika. Add the sugar, tomatoes and half the herbs. Cover and cook gently for 10 minutes.
Stir in the fennel, re-cover and cook for a further 5 minutes. Season, garnish with the remaining herbs and serve with crusty bread or couscous, if liked.