• Position the oven rack in the center of the oven. Preheat the oven to 400°F and butter a 1 1/2-quart baking dish.
  Butter for the baking dish
• Trim the fennel bulbs, cutting off the long green stalks where they meet the bulb and slicing a thin piece from the root base. Trim away any browned, bruised, or tough spots from the outer layers, but don’t remove the entire outer layer unless it’s badly damaged. Cut the bulbs in half from top to bottom, then cut out the small woody core, the little triangle at the base of each bulb. Place the halves cut-side down and slice thinly, about 1/8- to 1/4-inch thick. A mandoline does a great job, but you can slice the fennel by hand with a sharp knife if you don’t have one.
  2 large fennel bulbs
• In a large sauté pan or deep skillet, combine the sliced fennel, heavy cream, and wine. Add the grated garlic and nutmeg. Place the pan over medium heat and bring to a simmer. Reduce the heat to low and simmer, stirring frequently, for about 3 minutes or until the fennel begins to soften. Taste and season with kosher salt and freshly ground black pepper.
  1 cup heavy cream | 3 tbsp dry white wine | 1 clove garlic | 1/8 tsp freshly grated nutmeg | Kosher salt and freshly ground black pepper
  
• Using a slotted spoon, transfer the fennel to the buttered baking dish. Pour the cream into the baking dish until it comes nearly to the top of the fennel, but doesn't cover it.
  
• Bake the fennel in cream sauce for 20 minutes.

Prepare the Breadcrumbs

• Meanwhile, in a medium bowl, toss the breadcrumbs with the Parmesan cheese, thyme leaves or parsley, if using, and melted butter until the crumbs are well-coated.
  3/4 cup fresh breadcrumbs | 1/3 cup grated Parmesan cheese | 2 tbsp melted unsalted butter | 1 to 2 tsp thyme leaves or chopped fresh parsley (optional)
  
• After the gratin has cooked for 20 minutes, remove it from the oven and cover the top with the breadcrumb mixture.
  
• Return the gratin to the oven and bake for 10 to 15 minutes longer or until the breadcrumbs are golden brown and the casserole filling is bubbling around the edges. The fennel should be tender but not mushy.
  
• Let the gratin rest for 10 minutes, then garnish with some fennel fronds and chives, if desired.
  Fennel fronds | Chopped chives (optional)