Take a 9x13-inch baking dish and grease it really well with butter or cooking spray. Add the chunks of Brioche (1 loaf) to the tray and press it lightly into an even layer.
Evenly arrange the Peach (4) on top of the bread.
In a medium bowl, whisk the Egg (6), Heavy Cream (1 cup), Ground Cinnamon (1 teaspoon), Ground Ginger (1 teaspoon), and Vanilla Extract (1/2 teaspoon) together until smooth.
Pour it over the bread and peaches and lightly press everything down so it all gets glued together by the custard. Cover it and refrigerate it for at least an hour or up to overnight.
When you are ready to bake it, preheat the oven to 350 degrees F (175 degrees C).
Make the crumb topping. Put the Butter (4 tablespoon) in a small pot and heat it over medium heat. Let it gently melt and sizzle until it becomes nutty and slightly brown.
Stir the brown butter with the Dark Brown Sugar (3/4 cup) and All-Purpose Flour (1/2 cup) until it becomes a moist crumb.
Sprinkle it all over the top of the casserole, then bake it for 45 minutes or so, until the custard is set and the crumb is golden.
Let it cool for a few minutes or so when it's done, then serve immediately!