Bring a large saucepan of water to the boil. Trim the fennel, chopping off the bases and pulling away the thicker outer layers. Cut the bulbs into wedges, drop into the pan and boil for about 5 minutes until tender. Drain well.
Melt the butter with the garlic in a separate large pan over a low heat. Remove from the heat, add the fennel wedges and stir until coated with the butter. Add the reserved fronds and season with grated nutmeg, salt and pepper, to taste.
Transfer the fennel mixture to an ovenproof dish. Stir the breadcrumbs and Parmesan together and sprinkle over the top. Bake in the oven for 15-20 minutes until golden.