Step 1

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

Step 2

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

Step 3

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

Step 4

Make the custard. 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.

Step 5

Pour custard over bread and refrigerate. Pour the custard evenly over the bread, then press the bread down slightly into the custard. Cover with plastic wrap and refrigerate overnight or up to 24 hours. Or to bake right away, let sit for 1 hour at room temperature to give the bread time to absorb the custard.

Step 6

Mix the dry ingredients for the crumble topping. Toss 2 ounces chopped nuts (about 1/2 cup), 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.

Step 7

Add the butter to the crumble topping. Add the softened butter, and pinch and squeeze to incorporate the butter into the mixture until it forms moist, small clumps. Cover the crumble topping with plastic wrap and refrigerate until ready to bake.

Step 8

Heat the oven to 350°F. 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.

Step 9

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

Step 10

Cool the casserole. Remove from the oven and cool for 5 minutes before serving. Top with warm maple syrup or powdered sugar, and fresh berries.