Gratin:

Lightly oil shallow 2 qt glass baking dish. Heat olive oil in large wide pot over medium heat.

Add onion and garlic; sautee until soft but not brown, about 5 mins. Add fennel; increase heat to medium high and sautee until fennel is slightly softenedand beginning to brown, stirring freq, about 18 mins. Stir in broth, sherry, parsley, thyme, salt and pepper; simmer until most of liquid is absorbed, about 5 mins.

Transfer to prepared baking dish.

Crumb Topping:

Melt butter in large non stick skillet over medium heat. Add panko; sautee until golden, about 3 mins. Remove from heat; cool to room temp. Stir Pecorino, parsley and peel into panko. Preheat oven to 425 deg F

Sprinkle panko mixture over fennel.

Bake until gratin is heated through and topping is deep golden, about 20 minutes, serve warm.