Grease a 9x13-inch baking dish generously, then spread the cubed brioche or challah bread evenly across the bottom.

In a large bowl, whisk together the eggs, milk, heavy cream, granulated sugar, vanilla extract, ground cinnamon, nutmeg, and a pinch of salt until smooth. Pour this mixture evenly over the bread cubes.

In a separate bowl, combine the peach slices with brown sugar, cornstarch, lemon juice, and cinnamon. Gently toss to coat, then distribute the peach mixture evenly over the soaked bread cubes.

In a clean bowl, mix together the flour, brown sugar, cinnamon, and a pinch of salt. Cut in the cold, cubed unsalted butter until the mixture forms a crumbly texture. Sprinkle generously over the peach layer.

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

Preheat your oven to 175°C (350°F). Bake uncovered for 40-45 minutes, until golden brown and set in the center. Let cool for 10 minutes before serving with powdered sugar, maple syrup, or optionally whipped cream or vanilla ice cream.