Prep your dish: First things first! Grease a 9x13-inch baking dish with butter. Spread your bread cubes evenly inside. It might look like a lot of bread, but don't worry, it'll settle. Now, sprinkle about half of your blueberries over the bread.
Make the custard: In a large bowl, whisk together the eggs, milk, heavy cream, granulated sugar, vanilla, cinnamon, nutmeg, and that little pinch of salt. Keep whisking until it’s all smooth and well combined.
Soak it all up: Carefully pour the egg mixture evenly over the bread cubes. Gently press down on the bread with a spatula to make sure every piece gets a chance to soak up that delicious custard. Scatter the rest of the blueberries on top. Now for the hard part... cover the dish and let it sit in the fridge for at least 4 hours, or even better, overnight! This is the secret to a perfect, custardy casserole.
Get ready to bake: When you're ready to bake, preheat your oven to 350°F (175°C). While it's heating up, let's make the crumble topping.
Mix the crumble: In a separate bowl, mix the flour, brown sugar, and cinnamon. Add the cold butter pieces and use your fingertips or a pastry cutter to work the butter into the mixture until it looks like coarse crumbs. Sprinkle this heavenly mixture all over the top of your casserole.
Bake to perfection: Pop it in the preheated oven and bake, uncovered, for about 45-55 minutes. You'll know it's done when the top is golden brown and the center is set (a knife inserted into the middle should come out clean). Let it cool for about 10 minutes before serving. It's amazing with a drizzle of maple syrup!