In a baking dish, combine eggs, milk, Grand Marnier, orange and lemon zest, orange juice, sugar, and salt. Beat with a fork until well combined.
Using a serrated knife, slice the brioche 1/4-inch-thick. Dip the brioche slices into egg mixture and moisten thoroughly, being careful not to oversoak the bread.
Line a baking sheet with paper towels and set aside. Heat 1/4 cup oil and 1 tablespoon butter in a large non-stick skillet over medium high heat. Repeat in a second skillet. Transfer soaked bread to skillets and cook until deep golden brown, about 4 minutes per side. Transfer to paper towels to drain. Sprinkle with confectioners' sugar and serve hot with maple syrup.