Heat oven to 200C/180C fan/gas 6. Put the fennel and onion in a large roasting tin and toss with 2 tbsp oil, 1 tbsp lemon juice and plenty of seasoning. Roast for 20 mins until almost tender and starting to caramelise. Meanwhile, put the couscous in a medium bowl. Mix the saffron and hot stock, pour over the couscous and cover with cling film.
Rinse the fish with cold water inside and out, then pat dry. Slash deeply on both sides and season with salt, pepper and a pinch of the lemon zest. After 10 mins soaking, fluff up the couscous with a fork and add 2 tbsp lemon juice, the remaining zest, the dill, 1 tbsp pine nuts and seasoning to taste. Stuff the couscous into the cavity of each fish, reserving any leftover couscous for later.
Stir the tomatoes and currants into the tin. Sit the fish on top and roast for about 30 mins, depending on its size. Look for juices running from the slashes in the flesh. Tug gently on the back fin – if it comes away easily, the fish is cooked.
4 Scatter with the rest of the pine nuts, then serve from the tin, with lemon wedges for squeezing over. Serve one stuffed fish per person, or if you have cooked large fish, gently lift the fillets from the top of the fish first, then turn over and repeat. Share out the couscous stuffing and leftover couscous.