PreparationGratin: Lightly oil shallow 2-quart glass or ceramic baking dish. Heat 5 tablespoons oil in large wide pot over medium heat. Add onion and garlic; sauté until soft but not brown, about 5 minutes. Add fennel; increase heat to medium-high and sautéuntil fennel is slightly softened and beginning to brown, stirring frequently, about 18 minutes. Stir in broth and next 4 ingredients. Reduce heat to medium-low; simmer until most of broth is absorbed, about 5 minutes. Transfer to dish.DO AHEAD: Can be made 1 day ahead. Cool slightly; cover and chill. Let stand at room temperature 1 hour before baking. Crumb topping: Melt butter in large nonstick skillet over medium heat. Add panko; sauté until golden, about 3 minutes. Remove from heat; cool to room temperature. Stir Pecorino, parsley, and peel into panko. DO AHEAD: Can be made 1 day ahead. Store airtight in refrigerator. Preheat oven to 425°F. Sprinkle panko mixture over fennel. Bake until gratin is heated through and topping is deep golden, about 20 minutes. Serve warm.
PreparationGratin: Lightly oil shallow 2-quart glass or ceramic baking dish. Heat 5 tablespoons oil in large wide pot over medium heat. Add onion and garlic; sauté until soft but not brown, about 5 minutes. Add fennel; increase heat to medium-high and sautéuntil fennel is slightly softened and beginning to brown, stirring frequently, about 18 minutes. Stir in broth and next 4 ingredients. Reduce heat to medium-low; simmer until most of broth is absorbed, about 5 minutes. Transfer to dish.DO AHEAD: Can be made 1 day ahead. Cool slightly; cover and chill. Let stand at room temperature 1 hour before baking. Crumb topping: Melt butter in large nonstick skillet over medium heat. Add panko; sauté until golden, about 3 minutes. Remove from heat; cool to room temperature. Stir Pecorino, parsley, and peel into panko. DO AHEAD: Can be made 1 day ahead. Store airtight in refrigerator. Preheat oven to 425°F. Sprinkle panko mixture over fennel. Bake until gratin is heated through and topping is deep golden, about 20 minutes. Serve warm.