Combine brown sugar, butter and karo syrup in a saucepan, cook over moderate heat and bring to a slow boil.
Blend eggs, half and half, Grand Marnier, salt, nutmeg, cinnamon together.
Slice a loaf of thin crusted French bread into 1.5 inch wide sections.
Place sliced bread into Grease a 9 by 13 inch glass baking dish. Pour butter/sugar mixture to cover the bottom. Lay the French bread on top of this. Pour egg mixture over top to cover completely. Sprinkle cinnamon. Refrigerate overnight.
Set oven temp at 350 degrees. Place in oven and cook for 30-35 minutes, just until the egg sets. Do not overcook or you'll be disappointed! To serve, cut the toast into pieces and flip contents of the dish over onto a cookie sheet so that the brown sugar mixture is on top.