In an 8-inch skillet over medium-high heat, toast the bread crumbs, stirring constantly, until golden brown, about 2 minutes.
Set aside to cool.
In a 12-inch saute pan, heat the oil over medium heat, and swirl to coat the pan.
Saute the garlic and onion until soft, but not brown, about 3 minutes.
Add the fennel and continue sauteing, stirring frequently, until the fennel has softened and is beginning to brown, about 5 minutes.
Add the tomatoes, salt, and season with pepper to taste.
Lower the heat to medium-low and cook, stirring frequently, for 5 minutes longer.
Transfer to a shallow oven-to-table casserole or gratin dish.
Preheat the oven to 425°F.
In a medium bowl, combine the bread crumbs; Parmesan, and lemon zest.
Sprinkle evenly over the fennel mixture. (The gratin can be made up to this point 6 to 8 hours ahead.
Cover, and set aside at room temperature.) Bake the gratin until heated through and the topping is crisp, about 20 minutes.