Place 4 tablespoons unsalted butter on the counter to soften slightly.

Coat a 9x13-inch baking dish or other 3-quart baking dish with cooking spray and set aside.

Tear 1 loaf sourdough bread into bite-sized (1 to 2-inch) chunks. Arrange the bread in an even layer in the prepared baking dish.

Whisk 8 large eggs, 3 cups half and half (or 1 1/2 cups whole milk and 1 1/2 cups heavy cream), 1/2 cup packed light brown sugar, 1 Tablespoon vanilla extract, 1 teaspoon ground cinnamon, 1/2 teaspoon of the kosher salt, and 1/4 teaspoon of the ground nutmeg together in a large bowl until smooth and combined.

Pour the egg mixture evenly over the bread. Cover with plastic wrap and refrigerate overnight.

Mix 1/2 cup chopped nuts, 1/4 cup all-purpose flour, remaining 1/4 cup packed light brown sugar, remaining 1/4 teaspoon ground nutmeg, and remaining 1/4 teaspoon kosher salt together in a large bowl.

Add the softened butter to this mixture, working together with your hands if needed to incorporate the butter until it forms moist, small clumps. Cover the crumble topping with plastic wrap and refrigerate until ready to bake.

When ready to bake, arrange a rack in the middle of the oven and heat the oven to 350°F. Take the casserole out of the refrigerator, uncover, and set aside at room temperature for 30 minutes to take the chill off while the oven heats.

Sprinkle the crumble topping evenly over the casserole. Bake until the casserole is puffed, golden brown, and set, 45 to 50 minutes.

Remove from the oven and cool for 5 minutes before serving. Sprinkle with powdered sugar and serve with maple syrup.