Combine brown sugar and butter in a saucepan over medium heat. Stir until sugar has dissolved and mixture is smooth, about 5 minutes.
Pour caramel mixture into the bottom of a 9x13" baking dish (caramel layer should be about ¼" thick) Add baguette slices, pressing them into caramel.
In a medium mixing bowl, combine eggs, orange juice, cream, milk, cinnamon, and orange zest. Pour mixture over baguette slices. Cover, and chill in the fridge for at least 4 hours to overnight.
Preheat oven to 375°F. Remove french toast from fridge, and allow to sit at room temperature while oven preheats. Bake for 25-27 minutes or until french toast is golden brown.
When serving, flip so caramel layer is on top. Serve with favorite french toast toppings.
Notes
For best results, use day old baguette. If baguette is fresh, slice and place in the oven at 200°F until dried.