Place the wine and 150ml cold water in a large saucepan and bring to the boil. Add the mussels, cover and cook for 4 to 5 minutes, shaking the saucepan a few times until the mussels have opened. Drain the mussels and reserve the cooking liquid. Discard any mussels that have not opened.
Remove most of the mussels from their shells, reserving about 20 in their shells for the top of the cooked dish.
Meanwhile, heat the oil in a heavy-based saucepan over a medium heat. Add the fennel and onion and cook for 5 minutes until softened but not browned. Add the rice to the onion and fennel mixture and stir to coat with oil. Add the garlic and saffron and stir well.
Measure the reserved cooking liquid and add enough water to bring it up to 900mls. Start adding the liquid, a little at a time, while stirring all the time. Wait until each addition of liquid has evaporated before adding more. Keep going until all but a spoonful remains. Season to taste, then turn off the heat. Add the mussels and butter, stir, then cover with a lid for 5 minutes. Add the remaining tablespoon of cooking liquid, leaving any grit behind and fold in the chopped dill.