Coat a 9x13-inch baking dish with 1 tablespoon softened butter or nonstick cooking spray. Heat oven to 350ºF (177ºC).
In a large bowl, whisk together eggs, milk, ¼ cup maple syrup, vanilla, 1 teaspoon cinnamon, ½ teaspoon nutmeg and ¼ teaspoon salt. Set aside.
Slice peeled and cored apples into ¼-inch thick slices.
Melt 2 tablespoons butter in a large sauté pan over medium heat. Add apples, brown sugar, remaining ¼ teaspoon cinnamon, remaining ½ teaspoon nutmeg, cardamom and ⅛ teaspoon salt. Cook, stirring frequently, until apples are nearly fork-tender and syrup is thickened; 8–10 minutes.
Dip each slice of challah into custard mixture, flipping, for about 5 seconds.
Arrange soaked slices at an angle in an overlapping pattern in prepared baking dish. Pour any remaining custard over bread.
Layer cooked apple slices between layered bread slices.
Bake until puffy, about 25 minutes.
Meanwhile, add remaining 3 tablespoons butter and ⅓ cup maple syrup to skillet. Cook over medium heat, stirring frequently, until sugar syrup has thickened and leaves a trail when spatula runs through (and takes 3 seconds to fill in), about 2 minutes. Remove from heat and stir in walnuts and ⅛ teaspoon salt.
Remove baking dish from oven, pour caramel over french toast and continue baking until top is golden brown and center is set, about 25 minutes more.
Serve with warm maple syrup or a dusting or powdered sugar.