Instructions

1. Arrange a rack in the upper third of the oven and heat the oven to 400ºF.
2. Prepare the following, adding each to the same extra-large bowl or extra-large pot as you complete it: Trim the fronds from 1 1/2 pounds fennel. Halve the bulbs lengthwise through the root. If the bulbs are large, halve each piece again. Thinly slice the fennel about 1/4-inch thick (about 4 cups). Peel and thinly slice 1 pound carrots on a slight diagonal (about 2 1/2 cups). Halve 1 large leek lengthwise and rinse between the layers to clean. Thinly slice the white and light green parts crosswise.
3. Thinly slice 1 garlic clove and add it to the bowl; finely grate 1 garlic clove and reserve for the topping. Pick the leaves from 1 bunch fresh thyme until you have 2 tablespoons; add 1 tablespoon to the bowl and place the remaining 1 tablespoon in a medium bowl. Finely grate 3 ounces Parmesan cheese (about 1 1/2 cups); add 1 cup to the bowl and gently toss until combined; reserve the remaining cheese for the topping.
4. Place 1 1/2 cups heavy cream, 4 tablespoons of the unsalted butter, 2 teaspoons of the kosher salt, 1/4 teaspoon of the black pepper, 1/2 teaspoon grated nutmeg, and 1/4 teaspoon red pepper flakes in a small saucepan over low heat. Cook, stirring, until the butter just melts, about 5 minutes.
5. Pour the cream mixture over the fennel mixture and stir until the vegetables are coated. Transfer to a 9x13-inch or 3-quart baking dish and spread into an even layer. Cover the baking dish with aluminum foil. Bake for 20 minutes. Uncover and continue to bake until the fennel is tender and the top is beginning to darken, 25 to 30 minutes more. Meanwhile, prepare the topping.
6. Coarsely chop 1 1/2 cups pecans and add it to the bowl with the thyme. Add 1 1/2 cups fresh sourdough breadcrumbs, the remaining 1 teaspoon kosher salt, and 1/4 teaspoon black pepper. Melt the remaining 2 tablespoons unsalted butter in the microwave or on the stovetop. Add the reserved grated garlic to the butter and stir to combine. Pour the garlic butter over the breadcrumb mixture and toss until evenly coated.
7. Sprinkle half the remaining cheese on the gratin. Sprinkle all of the topping over the cheese, then sprinkle with the remaining cheese. Bake until the topping is deep golden-brown, 12 to 15 minutes more. Let rest 10 minutes before serving.