Place brioche pieces in the baking pan, layering generously with 3/4 of the total blueberries.
In a medium bowl, mix the sugar, cinnamon, vanilla, and maple syrup together. Mix in the beaten eggs.
Slowly pour in the milk and cream, mixing until well combined.
Pour or spoon the custard mixture over the bread and blueberries, making sure to coat all of the bread.
Cinnamon Sugar Topping
Stir together the sugar, flour, cinnamon and nutmeg in a small bowl. Add the melted butter and mix lightly to make a sugary crumble.
Sprinkle the crumble over the casserole and top with the remaining blueberries.
Soak the Casserole
Cover the casserole with plastic wrap and chill for at least 1 hour in the fridge. Or refrigerate overnight. This soaking step, allows the bread to absorb the custard.
Bake Blueberry French Toast
While the casserole is chilling, preheat the oven to 350℉ (180℃).
Remove casserole from the fridge, discard plastic wrap. Place the casserole on the middle rack of the oven and bake for 45-50 minutes. The bread and topping should be puffed and golden brown with a slight crunchiness on the edges. The blueberries will be juicy and bubbling.
Serve warm with a drizzle of maple syrup and a little whipped cream.