Preheat oven to 350 degrees. Line an 8x8 baking dish with parchment paper.
To an immersion blender cup or bowl, add your milk, eggs, protein powder, vanilla extract, cinnamon, and salt. Blend until smooth and set aside.
Cut your sourdough bread into bite-size pieces. Toss the bread cubes into your 8x8 baking pan in an even layer.
Dice your apple or other fruit into small pieces. Add to the baking dish on top of the bread and toss lightly with your hands to mix.
Take your milk & egg mixture and pour it evenly across the whole baking dish, making sure to get all of the bread wet. Use the back of a spatula to press down on the bread as necessary to help it soak up the liquid evenly.
Sprinkle your pumpkin seeds on top. Place in the oven to bake for 45-50 minutes. The casserole is done when you press on the center of the casserole with the back of a spoon and it doesn't seep any of the liquid.
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Notes
Blueberries are also another great choice for added fruit.