
For the French toast: Grease a 13-by-9-inch baking pan with butter. Tear the bread into chunks, or cut it into cubes, and evenly distribute it in the pan. Crack the eggs in a big bowl. Whisk together the eggs, milk, cream, granulated sugar, brown sugar, and vanilla. Pour evenly over the bread. Cover the pan tightly and store in the fridge until needed (overnight preferably). Or you can make it and bake it right away—it's delicious no matter what!
For the topping: Mix the flour, brown sugar, cinnamon, salt, and some nutmeg in a separate bowl. Stir together using a fork. Add the butter and use a pastry cutter to mix it all together until the mixture resembles fine pebbles. Store in a plastic bag in the fridge.
When you're ready to bake the casserole, preheat the oven to 350°F. Remove the casserole from the fridge and sprinkle the topping over the top. Bake for 45 minutes for a softer, bread pudding-like texture or for 1 hour or more for a firmer, crisper texture.
Scoop out individual portions. Top with butter, drizzle with warm syrup, and sprinkle with blueberries.