Preheat the oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or nonstick spray.

Cut the loaf into 1-inch cubes. Spread the cubes in an even layer in the prepared dish.

In a medium bowl, whisk the eggs, milk, granulated sugar, vanilla, cinnamon, and nutmeg until smooth and combined.

Pour the custard evenly over the bread cubes, pressing down lightly so the bread soaks the mixture. Let sit for 10 to 30 minutes — longer if your bread is very fresh.

In another bowl, mix the brown sugar, oats, flour, and the optional chopped nuts. Add the cold cubed butter and use a fork or pastry cutter to combine until the mixture is coarse and crumbly.

Sprinkle the crumble evenly over the soaked bread.

Bake for 30 to 40 minutes, until the top is golden, the crumble is crisp, and the center is set (a knife inserted in the middle should come out mostly clean).

Let the casserole rest for about 5 minutes so it firms up slightly. Serve warm with maple syrup, fresh fruit, or a dusting of powdered sugar.