Step 1Toast the bread crumbs in an 8-inch skillet over medium-high heat, stirring constantly, until golden brown, about 2 minutes. Set aside to cool.
Step 2Heat the oil in a very large (16-inch) skillet over medium heat, and swirl to coat the pan. Cook the garlic and onion until soft, but not brown, about 5 to 6 minutes. Add the fennel and continue cooking, stirring frequently, until the fennel has softened and is beginning to brown, about 20 to 25 minutes. Add the tomatoes, salt and pepper to taste. Reduce the heat to medium-low and cook, stirring frequently, for 5 minutes longer. Transfer to a shallow 3-quart oven-to-table casserole or gratin dish.
Step 3Heat the oven to 425 degrees.
Step 4Combine the bread crumbs, Parmesan and lemon zest in a bowl and sprinkle evenly over the fennel mixture. (The gratin can be made up to this point 6 to 8 hours ahead. Cover and refrigerate, then bring to room temperature before baking.)
Step 5Bake the gratin until heated through and the topping is crisp, about 20 minutes. Serve immediately.