From:

http://sallysbakingaddiction.com/2015/03/25/unbelievable-blueberry-french-toast-casserole/

Casserole:

    12-14 oz bread, cubed.
    1 cup (180g) fresh or frozen blueberries
    8 large Eggland's Best eggs
    2 and 1/4 cups (540ml) whole milk
    1/2 teaspoon ground cinnamon
    3/4 cup (150g) packed light brown sugar
    1 Tablespoon (15ml) vanilla extract

Streusel Topping (optional:  too rich.)

    1/3 cup (69g) packed light brown sugar
    1/3 cup (42g) all-purpose flour (spoon & leveled)
    1/2 teaspoon ground cinnamon
    6 Tablespoons (86g) unsalted butter, cold and cubed


    Grease a 9x13 pan with butter or spray with nonstick spray. Slice
    then cut the bread into cubes, about 1 inch in size. Spread cubes
    into the prepared baking pan and top evenly with blueberries. Set
    aside.

    Whisk the eggs, milk, cinnamon, brown sugar, and vanilla together
    until no brown sugar lumps remain. Pour over the bread. Cover the
    pan tightly with plastic wrap and stick in the refrigerator for 3
    hours - overnight. Overnight is best.

    Preheat oven to 350°F (177°C). Remove pan from the refrigerator.

    Prepare the topping: Whisk the brown sugar, flour, and cinnamon
    together in a medium bowl. Cut in the cubed butter with a pastry
    blender or two forks. Sprinkle the topping over the soaked bread.

    Bake for 45-55 minutes or until golden brown on top. I usually
    bake it for 45 minutes because I like it softer. Serve
    immediately. Cover leftovers tightly and store in the refrigerator
    for 2-3 days.

    Make ahead tip: Prepare the topping in advance, cover tightly, and
    store in the refrigerator. Sprinkle over the soaked bread before
    baking. For freezing, prepare the recipe through step 4 (without
    preheating the oven) and freeze for up to 2 months. Thaw overnight
    in the refrigerator then bake as directed.