Coat a 9- x 13-inch baking dish with nonstick cooking spray.
Cut the slices of bread into 1-inch cubes. Place half of the bread cubes in the prepared baking dish and sprinkle evenly with the blueberries. Drop small pieces of mascarpone over the top of the blueberries and top with the remaining bread cubes.
In a medium mixing bowl, whisk together eggs, milk, sugar, vanilla extract, almond extract, cinnamon, and nutmeg until well combined. Pour the mixture evenly over the top of the bread cubes. Cover the dish with plastic wrap and refrigerate overnight.
The next morning, remove the casserole from the refrigerator and allow the dish to sit on the counter while you preheat the oven to 350 degrees F.
Combine the melted butter, brown sugar, and almonds in a small mixing bowl. Sprinkle the mixture evenly over the top of the casserole and bake, uncovered, 40 to 45 minutes, or until casserole is golden brown on top. If needed, you can broil the top briefly to further brown the crust. Let it rest 10 minutes before serving.