Slice bread into 1-1/2-inch slices. Spray a 9x13” pan with cooking spray. Place bread in baking dish with sides touching (snug fit).
In a bowl with a whisk, beat together eggs, 1/2 c. sugar, milk and vanilla for about 30 seconds. Pour one half of egg mixture over bread. Peel, core and slice apples. Place apples on top of bread to cover. Pour rest of egg mixture evenly over bread.
Mix remaining 1/2 c. sugar with cinnamon and nutmeg and sprinkle evenly over top of apples. Dot with butter. Cover and refrigerate overnight.
In morning, preheat oven to 350 degrees. Uncover dish and bake in oven for 1 hour or until center is set. Remove from oven and allow to rest for 5 to 10 minutes before serving. Cut into squares and serve with warm syrup or powdered sugar.