Preheat your oven to 350°F. Grease a 9x13-inch baking dish with butter or non-stick spray. Spread cubed brioche or challah bread across the bottom of the dish.

In a large mixing bowl, whisk together 6 large eggs, 2 cups of whole milk, and 1 cup of heavy cream until smooth. Add in ¾ cup of granulated sugar, 2 teaspoons of vanilla extract, 1 teaspoon of ground cinnamon, a pinch of ground nutmeg, and ½ teaspoon of salt. Pour this custard over the bread cubes.

In another bowl, toss 4 cups of peach slices with ¾ cup of brown sugar, 1 tablespoon of cornstarch, 1 tablespoon of lemon juice, and ½ teaspoon of cinnamon. Layer the peach filling over the custard-soaked bread.

In a separate bowl, mix 1 cup of flour, ½ cup of brown sugar, ½ teaspoon of ground cinnamon, and a pinch of salt. Cut in ½ cup of cold unsalted butter until the mixture resembles coarse crumbs. Sprinkle it over the peach layer.

Cover the dish with plastic wrap and refrigerate overnight or allow to sit at room temperature for 30 minutes.

Bake for 40-45 minutes until the top is golden brown and the custard has set. Cool for 10 minutes before serving. Dust with powdered sugar.