03Cook the broth over medium-high heat until it is reduced by a little more than half, to approximately 1 1/4 cups. Force the broth through a medium sieve into a bowl, using a spatula to push through the garlic cloves.04Preheat the oven to 350 degrees. Cover the bottom of an 8-inch baking dish or gratin dish with several spoonfuls of the strained broth. Make a layer of fennel and potatoes, arranging the slices so that fennel alternates with potato; use up all the fennel. Spoon on a little more broth and sprinkle with one-third of the Parmesan cheese and some freshly ground black pepper. Put the remaining potato slices on top - they should overlap slightly.05Lightly press the potatoes with the palm of your hand so that the layer is even. Pour the remaining broth over the potatoes, grind pepper on top and sprinkle with the rest of the Parmesan cheese. Bake for an hour and 15 minutes.06While the gratin is baking, saute the bread crumbs in the butter until they are golden-brown. Five minutes before the gratin is done, sprinkle the bread crumbs over the top with about a tablespoon of the chopped fennel leaves. Let the dish cool slightly before serving.