Preheat oven to 450°F.
Arrange croissants on a large rimmed baking sheet and toast in the oven for about 3-4 minutes.
Place croissants in a greased 9 x 13-inch baking dish.
Sprinkle berries over top of the croissants.
In a large bowl, whisk together eggs, sugar, cinnamon, nutmeg, and salt until blended. Stir in the milk, orange juice, and vanilla. Pour liquid mixture over the croissants.
Allow casserole to sit on the counter for about 30 minutes so that the croissants can absorb some of the liquid and soften. As the croissants get softer, gently press down on them to submerge them more in the liquid. If you prefer, you can cover the dish and refrigerate the casserole for 8-12 hours before baking.
Adjust the oven temperature to 350°F.
Bake the casserole for a total of 60-70 minutes, adding a loose foil cover on top after the first 25 minutes to prevent excessive browning. The casserole is done when it's puffed, golden brown, and the center is set. If you insert a knife or cake tester into the middle of the casserole, it should come out clean.
Let the casserole rest for at least 10 minutes before slicing and serving.
Dust with powdered sugar and serve with a side of maple syrup for drizzling.