Heat oven to 375 degrees. Remove a thin layer of the fennel bulbs’ tough exterior with a paring knife or sharp vegetable peeler. Cut the fennel crosswise into half-inch-thick slices. Bring a large pot of salted water to boil. Boil the fennel for 1 minute, then put it in a bowl of cold water, drain and pat dry. Season with salt and pepper. Lightly oil an oven-proof baking dish. Layer in the fennel to a depth of 1 1/2 inches (pushing down, if necessary).
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In a small bowl, stir together 3 tablespoons olive oil, the fennel seed and the garlic, smashed to a paste with a little salt, the pepper flakes and the chopped rosemary. Drizzle 2 tablespoons of this mixture over the fennel. Sprinkle with the rosemary leaves. Cover with a layer of sliced or shredded mozzarella, then sprinkle with bread crumbs. Drizzle the remaining oil mixture, then sprinkle with Parmesan. (The dish may be prepared to this point several hours before baking.)
