Prepare your slow cooker by greasing it with butter or cooking spray.
In a large mixing bowl, whisk together the eggs, milk, sugar, vanilla extract, cinnamon, and salt until well combined.
Add the cubed brioche bread to the egg mixture, gently folding it until all pieces are coated.
Transfer the bread mixture to the prepared slow cooker, spreading it out evenly.
Cover and cook on low for 6-8 hours or high for 3-4 hours.
While the casserole is cooking, make the berry compote.
In a saucepan over medium heat, combine the mixed berries, sugar, and cornstarch (if using).
Cook the berry mixture, stirring occasionally until thickened, about 10-15 minutes.
Once the casserole is cooked, turn off the slow cooker and let it sit for 10 minutes before serving.
Serve the casserole warm, topped generously with the berry compote.