Preheat your oven to 350 degrees. This ensures that the casserole bakes evenly, giving you that perfect golden brown crust.
Cut the butter for the streusel topping into small pieces and leave it on the counter to soften. This way, it will blend easily with the dry ingredients later.
Spray a 9 x 11 casserole dish with some spray oil. This will help prevent the casserole from sticking and make cleaning easier!
In a medium-sized bowl, crack open the eggs and mix them with a whisk until they’re well blended. Add the milk, vanilla, and a pinch of salt before whisking again until combined.
Slice the bread into 1-inch cubes. You should have about 8 to 10 cups of bread when you’re done slicing. This size is perfect for soaking up the custard.
Add half of the bread cubes to the bottom of the casserole dish. Spread half of the blueberries over the bread to create a delicious layer.
Add the remaining bread on top of the blueberries. Pour the custard mixture evenly over the bread, pressing down any pieces that stick up above the custard. This ensures every bite is flavorful.
Top the casserole with the rest of the blueberries. Let the assembled dish sit for at least 30 minutes, or even better, store it in the fridge overnight.
In a medium-sized bowl, add the softened butter, flour, brown sugar, oats, cinnamon, and a pinch of salt. Using a fork or pastry cutter, mix until the mixture resembles wet sand.
If you prepped the casserole the night before, keep the streusel topping separate until morning. If you're baking it right away, sprinkle the streusel on top of the casserole.
Bake for 23 to 25 minutes until the casserole is gooey but not raw. You should see the mixture bubbling. Serve with a generous drizzle of maple syrup and enjoy!