Place the Croissants (2) cut sides down, in a shallow baking dish just large enough to fit them in one layer.
In a mixing bowl, whisk together the Milk (1 cup), Large Egg (3), Granulated Sugar (1/4 cup), Ground Cinnamon (1/2 tablespoon), Ground Nutmeg (1/8 tablespoon), zest of the Orange (1), Vanilla Extract (1/4 tablespoon), Grand Marnier (2 tablespoon), and Kosher Salt (1 pinch).
Pour mixture over the croissant halves. Soak the croissants for 10 minutes, using a spoon to ladle the custard over them halfway through.
Meanwhile, in a medium bowl, toss the Fresh Raspberries (1 cup) and Fresh Strawberry (1 cup) with the remaining Grand Marnier (2 tablespoon).
Melt half the Unsalted Butter (1/2 tablespoon) in a large sauté pan over medium-low heat. When the butter is hot, place both halves of one of the croissants, cut sides down, in the pan. Immediately sprinkle the outsides with granulated sugar and cook for 4 minutes.
Using a spatula, flip the croissant halves and cook for an additional 2-3 minutes, until golden brown.
Remove and repeat with the remaining croissant and Unsalted Butter (1/2 tablespoon). Serve immediately with the berries, Maple Syrup (to taste) and Whipped Cream (to taste), if using.