Generously grease a 9x13 inch (3-quart) baking dish with butter or non-stick spray.

Arrange the cubed bread in a single, even layer in the prepared baking dish. Sprinkle 1 1/2 cups of the blueberries evenly over the bread cubes, tucking some in between the pieces.

In a large mixing bowl, whisk together the large eggs until the yolks are broken. Add the whole milk, heavy cream, granulated sugar, vanilla extract, 1 tsp cinnamon, nutmeg, and salt. Whisk until the mixture is smooth and well combined.

Carefully and evenly pour the egg custard mixture over the bread and blueberries in the dish. Gently press down on the bread with a spatula to ensure all pieces are saturated with the custard.

Cover the dish tightly with plastic wrap or foil and refrigerate for at least 4 hours, or preferably overnight. This allows the bread to fully absorb the custard.

When ready to bake, preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and let it sit at room temperature for about 20-30 minutes.

While the oven preheats, prepare the streusel topping. In a small bowl, combine the all-purpose flour, packed light brown sugar, and 1 tsp cinnamon. Add the cold, cubed butter and use your fingertips or a pastry blender to cut the butter into the mixture until it resembles coarse crumbs.

Sprinkle the remaining 1/2 cup of blueberries over the top of the casserole, followed by an even layer of the streusel topping.

Bake uncovered for 45 to 55 minutes, or until the casserole is puffed, golden brown, and the center is set (a knife inserted in the center should come out clean). If the top begins to brown too quickly, you can loosely tent it with aluminum foil for the last 10-15 minutes of baking.

Let the casserole rest for 10 minutes before slicing and serving. Serve warm with maple syrup, powdered sugar, or a dollop of whipped cream.