Prep this two days before, then finish it off 45 minutes before the feast.
Arrange a rack in the center of the oven; preheat to 375°. In a large wide heavy pot, melt half the butter over medium-high heat. Add half the sliced fennel and half the leeks. Cook, stirring occasionally, until the fennel and leeks are crisp-tender, 9 to 11 minutes. Using a slotted spoon, transfer the fennel and leeks to a 9-by-13-inch baking dish. Repeat with the remaining butter, fennel, and leeks. Transfer the fennel and leeks to the baking dish. Stir in the thyme; season with salt and pepper. Spread the vegetables out in an even layer in the baking dish. Add the cream and stock. Using the back of a large spoon, press down on the vegetables to submerge them in the cream and stock and to smooth the top. Bake until the gratin is bubbling and the top is deep golden, 45 to 50 minutes.
In a medium bowl, mix the cheese, breadcrumbs, parsley, and fennel fronds.
Remove the gratin from the oven. Top with the breadcrumbs. Broil until the top is golden, 1 to 3 minutes. Let stand for 10 minutes before serving.
Up to 2 Days Ahead: Make the gratin through Step 1. Let cool, then cover and refrigerate.
45 Minutes Before Serving: Warm in a 375° oven until heated through, 20 to 30 minutes. Continue the recipe, starting with Step 2.
Thanksgiving recipes for planners and procrastinators here.