Grease a 9×13-inch pan or any 3.5—4-quart baking dish with butter or nonstick spray. Spread the cubed bread in the pan and sprinkle evenly with blueberries. Set aside.

In a large bowl, whisk the milk, eggs, heavy cream, brown sugar, vanilla, cinnamon, lemon zest, and salt until combined. Pour over the bread. Cover the pan tightly and refrigerate for 3–24 hours. Overnight is best.

In a medium bowl, mix the brown sugar, flour, and cinnamon. Add the cold butter. Using a pastry blender or two forks, cut the butter into the brown sugar mixture until pea-sized crumbles form. Cover and refrigerate the topping for at least 15 minutes or up to 1 day. Chilling it helps ensure that it won’t sink into the casserole.

Remove the casserole from the refrigerator and let it sit at room temperature for 10 to 15 minutes while you preheat the oven to 350°F (177°C).

Sprinkle the crumb topping evenly over the casserole. Bake uncovered for 20 minutes. Cover loosely with aluminum foil and continue to bake until the center appears set, 25 to 35 minutes more.

Cool the casserole for 5 minutes before serving. Store tightly covered in the refrigerator for up to 3 days.