Butter a 2 quart baking dish and set aside. Cut the bread into cubes and place into the dish.
In a mixing bowl, whisk together the eggs, cream, milk, sugar, vanilla, lemon zest and salt. Pour over the bread cubes.
Layer blueberries on top. Cover and chill for 2 hours or overnight.
Preheat the oven to 350º. Mix butter, brown sugar and flour until it reaches a crumbly consistency and layer on top of the casserole. Bake for 1 hour.
Mix powdered sugar and lemon juice to create a glaze. After the casserole bakes for an hour, allow it to cool for ~15 minutes before drizzling the glaze on top.