Preheat oven to 350 degrees F (175 degrees C). Grease a baking dish and sprinkle with bread crumbs.
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add fennel wedges, cover, and steam until tender but still crisp, about 10 minutes. Drain and transfer to the baking dish.
Melt butter in a large skillet over medium heat. Stir in flour to make a soft paste. Gradually whisk in 1 1/2 cup plus 3 tablespoons milk; simmer until sauce is creamy and thick, 5 to 6 minutes. Season with salt, pepper, and nutmeg.
Pour sauce over fennel in the baking dish. Sprinkle Parmesan cheese on top.
Bake in the preheated oven until golden, about 20 minutes. Broil until top is browned, 2 to 3 minutes. Let stand before serving, about 5 minutes.