Preheat your oven to 350°F (175°C) and lightly grease a large 9x13-inch baking dish with cooking spray.
Slice the whole grain bread into 1-inch cubes and spread them evenly across the bottom of the prepared baking dish.
Evenly distribute the mixed berries over the bread cubes, allowing them to settle into the gaps.
In a large mixing bowl, whisk together the liquid egg whites, 0% Greek yogurt, skim milk, maple syrup, vanilla extract, ground cinnamon, and sea salt until the mixture is smooth and well-combined.
Pour the custard mixture slowly over the bread and berries, ensuring every piece of bread is fully saturated.
Use a spatula to gently press the bread down into the liquid to help it absorb the custard evenly.
Place the dish in the oven and bake for 45 to 50 minutes, or until the custard is set and the top is golden brown and slightly crisp.
Remove from the oven and let the casserole rest for 5 minutes before slicing into 8 equal portions and serving.