Butter a 4 to 5 quart rectangular or oval baking dish and set aside.
In a large bowl, whisk together half and half, vanilla, eggs, brown sugar, cinnamon, nutmeg, and salt. Dip croissant halves into mixture until fully coated.
Arrange the bottom half of the croissants into the prepared casserole, overlapping slightly.
Add about 3/4 cup (total) of strawberries and blueberries on top of the croissant layer.
Top with the other half of the croissants and pour the remaining milk and egg mixture over the casserole. Press croissants into mixture to absorb.
Sprinkle the remaining berries over the top of the casserole and top with Jessica's Natural Foods Gluten - Free Vanilla Maple Granola. Cover the dish with plastic wrap, and refrigerate for at least 2 and up to 8 hours.
Preheat oven to 350 degrees F. And remove the casserole from the refrigerator 30 minutes before baking and let stand at room temperature.
Cover with foil and bake for 30 minutes. Remove cover and bake for an additional 15-20 minutes, until puffed and golden.
Let cool for 10 minutes. Dust with powdered sugar, cut into wedges, and serve with whipped cream, maple syrup and more berries.