Clean the fennel, remove the hard piece of root and cut into small pieces. Pluck the fennel green, cut into small pieces and set aside. Cook the fennel for about 10 minutes, then drain. Puree together with lemon juice, parmesan and mascarpone with a hand blender. Butter a baking dish, pour in the puree and sprinkle with breadcrumbs and flakes of butter.
Bake in a hot oven at 180 ° C top / bottom heat for about 10 minutes.