If your bread is not day-old, preheat the oven to 350°F (175°C). Spread the cubed bread on a baking sheet and toast for about 10 minutes until slightly dry; this step is optional but helps prevent a soggy casserole.
Spray a 9x13 inch baking dish with nonstick spray and evenly arrange the toasted or day-old bread cubes in the dish.
Scatter the blueberries evenly over the bread cubes.
In a large bowl, whisk together the liquid egg whites (or 8 whole eggs), almond milk, maple syrup, vanilla, cinnamon, nutmeg, and a pinch of salt until well combined.
Pour the egg-and-milk mixture evenly over the bread and blueberries, pressing gently so the bread absorbs the liquid. Cover and refrigerate for at least 1 hour or overnight for best results.
When ready to bake, preheat the oven to 350°F (175°C). Remove the cover and bake the casserole for 45 to 50 minutes, until set and golden on top.
Let the casserole rest a few minutes, then top with powdered sugar, maple syrup, or extra blueberries if desired and serve.