Butter 9 x 13 pan. Cut challah in 3/4 inch slices. Lightly butter the front of each slice. Layer bread like shingles to fill pan.
For custard, whisk eggs, whole milk, half and half, brown sugar, cinnamon, vanilla and salt till blended. Pour over bread slices in pan. Cover the pan with plastic wrap. Refrigerate overnight.
Preheat oven to 350 degrees.
For spiced apples, melt butter in fry pan on medium heat. Add sliced apples, brown sugar, maple syrup and cinnamon. Bring to slow boil for sauce to thicken. About 6 minutes. Add chopped pecans. Distribute evenly over top of bread. Cover with tin foil.
Bake for 60-65 minutes total. Remove foil after 30 minutes so apples caramelize.
Top with powdered sugar when ready to serve.
Keyword Apples, Breakfast, brunch, Custard, French Toast, Holiday Recipe, Make Ahead, Nuts, Overnight French Toast, Pecans, Spiced Apple, Whole Milk