Make the night before
In a saucepan, cook brown sugar, butter and syrup until thick (about 5-7 minutes). Stir in chopped walnuts, if using. Pour into an ungreased 15x10x2-inch baking pan. Arrange apple slices on top.
In a bowl, mix the eggs, milk and vanilla.
Dip bread slices into egg mixture and let soak for 1 minute.
Place bread slices on top of apples. Cover and refrigerate overnight.
In the morning, remove from refrigerator; let stand for 30 minutes before baking.
Bake uncovered at 350 degrees for 35-40 minutes.