Start by cubing your thick-cut bread into bite-sized pieces. If your bread isn’t slightly stale, toast the cubes in a preheated oven at 300°F (149°C) for about 10 minutes. Place the bread cubes in a large mixing bowl, and set aside.
In another bowl, whisk together the eggs and whole milk until well combined. Add in the cinnamon and whisk thoroughly until smooth.
Gently combine the cubed bread with the egg custard mixture. Fold in the blueberries and let the mixture rest.
For the streusel, mix together brown sugar, flour, cinnamon, and cold, diced unsalted butter to create a crumbly texture.
Transfer the soaked bread and blueberry mixture into a greased casserole dish, spreading it out evenly. Sprinkle the prepared streusel topping over it.
Cover the casserole and refrigerate overnight, or for at least three hours.
Preheat your oven to 350°F (175°C) and bake for about 45-50 minutes until golden brown.