In a large bowl, whisk together half and half (or milk), vanilla, eggs, brown sugar, cinnamon, nutmeg, and salt. Dip croissant halves into mixture until fully coated.
Arrange croissants into prepared casserole, overlapping. Pour the remaining milk and egg mixture over the casserole, gently pressing to absorb.
Tuck about 3/4 cup (total) of strawberries and blueberries between the croissant layers. Sprinkle the remaining berries over the top of the casserole. Cover the dish with plastic wrap (or the lid, if using a lidded baker), and refrigerate for at least 2 and up to 8 hours.
Preheat oven to 350 degrees F. Remove casserole from the refrigerator 30 minutes before baking and let stand at room temperature. If you used plastic wrap, uncover, and discard wrap.
Bake 30 minutes, covered with foil or the baking pan lid. Remove cover and bake an additional 15-20 minutes, until puffed and golden. Let stand at room temperature for 10 minutes. Dust with powdered sugar, cut into wedges, and serve with whipped cream and/or maple syrup.
Make the Rum Maple Syrup
Warm maple syrup. Stir in rum and drizzle over baked french toast wedges.