Ingredients

• 4 tablespoons (1/2 stick) unsalted butter, melted, divided
• 1 large fennel bulb with fronds
• 2 (12-ounce) bags frozen pearl onions, thawed
• Kosher salt
• 1 1/3 cups heavy cream
• Finely grated zest from 1 orange
• 1 cup panko (Japanese breadcrumbs) or coarse plain breadcrumbs
• 1/2 cup finely grated Parmesan
• 1/3 cup finely chopped flat-leaf parsley
• 1/4 teaspoon freshly ground black pepper
• 1/4 teaspoon red pepper flakes

Instructions

1. Position rack in middle of oven and preheat to 400ºF. Brush a 2-qt. gratin or baking dish, preferably oval or rectangular, with 1/2 Tbsp. butter.
2. Cut fennel bulb and stems into 1/4" pieces. Finely chop 2 Tbsp. fennel fronds; set aside.
3. Combine onions, fennel bulb and stems, 1/4 tsp. salt, and 3/4 cup water in a large saucepan. Bring to a boil over high heat, stirring occasionally. Reduce heat to medium, cover, and simmer 5 minutes. Drain onion mixture, pat dry, and transfer to prepared dish.
4. Bring cream and 1/2 tsp. salt to a boil in a small saucepan over medium-high heat, then pour over onion mixture. Bake 15 minutes.
5. Meanwhile, combine orange zest, breadcrumbs, cheese, parsley, black pepper, red pepper flakes, reserved fennel fronds, 1/2 tsp. salt, and remaining 3 1/2 Tbsp. melted butter in a medium bowl.
6. Remove onion mixture from oven and top with breadcrumb mixture. Return to oven and continue baking until breadcrumbs are deep golden brown and cream is bubbling around the edges, 25–30 minutes. Let rest 10 minutes before serving.