Heat the olive oil in a large, deep frying pan, add the onion and cook gently for 5 minutes or until softened. Meanwhile, soak the saffron in the boiling water.
Add the capsicums and garlic to the pan and cook for 3–4 minutes. Stir in the rice, then add the wine, hot stock, saffron mixture, rosemary sprigs and pepper. Bring to the boil, then cover, reduce the heat and cook for 25 minutes or until the rice is almost tender, adding more stock if it is all absorbed before the rice is ready.
Meanwhile, preheat the grill to medium. Place the red mullet in a shallow flameproof dish and grill for about 6 minutes on each side (you may have to remove the grill tray to fit the dish). To test if the fish is cooked, insert a knife tip near the bone; the flesh should flake easily. Cut off the heads and tails, remove the bones and cut the flesh (with skin) into large bite-sized pieces.
Add the asparagus tips and peas to the rice and mix in. Cook, covered, for a further 5 minutes, by which time all the stock should have been absorbed and the rice should be tender.
Add the prawns and fish to the pan and stir carefully to mix into the rice. Cover and cook gently for 4–5 minutes to heat through, then serve with lemon wedges.