Ingredients

• 4 tablespoons unsalted butter, plus more for pan
• 12 cups (1" pieces) brioche (from about 2 loaves)
• 1 small butternut squash (about 1 1/2 lb.), peeled, seeds removed, cut into 1/2" pieces
• 2 large leeks, white and pale green parts only, halved lengthwise, cut into 1/2" pieces
• 1 teaspoon thyme leaves
• 2 teaspoons kosher salt, plus more
• 4 large eggs
• 3 cups heavy cream
• 3 cups whole milk
• 1 teaspoon sugar
• 1 1/2 cups coarsely grated Emmenthal cheese, Comté, and/or aged Gouda, divided
• Crème fraîche and fennel fronds (for serving)

Instructions

1. Preheat oven to 350°F. Butter a 13x9" glass or ceramic baking dish; set aside. Spread out brioche on a large rimmed baking sheet and bake, tossing halfway through, until golden brown and crisp, 25–30 minutes. Let cool, then transfer to a large bowl.
2. Heat remaining 4 Tbsp. butter in a large skillet over medium. Cook squash, leeks, and thyme, stirring occasionally until squash is tender, 10–12 minutes. Mix in a pinch of cayenne; season with salt. Transfer to bowl with brioche.
3. Whisk eggs, cream, milk, sugar, 2 tsp. salt, and a pinch of cayenne in a large bowl to combine.
4. Add 1 cup cheese to bowl with brioche and toss to evenly distribute. Transfer to prepared baking dish and pour 5 cups egg mixture over; gently press bread into liquid to coat. Pour remaining egg mixture over and let sit 15 minutes.
5. Scatter remaining 1/2 cup cheese over bread pudding and bake until puffed and custard is set in the center, 60–75 minutes. Let cool at least 10 minutes before serving.