Preheat your oven to 375°F (190°C) and grease a 9x13-inch casserole dish.
In a large mixing bowl, whisk together the eggs, milk, sugar, vanilla extract, and cinnamon until well combined.
Tear the croissants into bite-sized pieces and layer them evenly in the greased casserole dish.
Pour the custard mixture over the croissant pieces, ensuring they're well-coated. Press down gently to help the croissants soak up the custard.
Sprinkle mixed berries evenly over the top of the croissant mixture.
Cover the dish with plastic wrap and refrigerate for at least one hour, or preferably overnight to allow the flavors to develop.
Once ready to bake, remove the plastic wrap and cover the dish with aluminum foil. Bake for 25 minutes.
After 25 minutes, remove the foil and bake for an additional 30-35 minutes, or until the top is golden brown and the custard is set.