Lightly grease a 9x13-inch (or 3-quart) baking dish with unsalted butter. Tear the unsliced bread loaf into bite-sized chunks, about 1 to 2 inches in size, and arrange them evenly in the prepared baking dish.

In a large bowl, whisk together the eggs, half-and-half (or the combination of milk and cream), brown sugar, vanilla extract, ground cinnamon, kosher salt, and ground nutmeg until smooth and well combined.

Pour the prepared custard mixture evenly over the bread chunks in the baking dish. Gently press the bread down so it absorbs the liquid. Cover the dish and refrigerate overnight or up to 24 hours. If you prefer to bake right away, let the dish sit at room temperature for an hour to allow the bread to soak up the custard.

In a large bowl, combine the chopped pecans or walnuts, all-purpose flour, remaining packed light brown sugar, remaining ground nutmeg, and remaining kosher salt. Add the room-temperature butter and use your fingers to pinch and squeeze the mixture until moist, small clumps form. Refrigerate the crumble topping until ready to use. I find refrigerating the crumble before baking helps it form the best texture.

When ready to bake, preheat the oven to 350°F (175°C). Remove the casserole from the refrigerator and let it sit at room temperature, uncovered, for 30 minutes to remove the chill. Sprinkle the prepared crumble topping evenly over the casserole. Bake uncovered for 45 to 50 minutes, or until puffed, golden-brown, and set in the center.

Let the baked casserole cool for 5 minutes before serving. Serve slices with a drizzle of warm maple syrup or a dusting of powdered sugar, and optionally, fresh berries on the side. For extra indulgence, I love to serve mine with a generous pour of maple syrup — it really brings out the flavors!