Preheat oven to 250 degrees F.In a 9"X13" baking dish, or any dish large enough to lay all bread slices out in a single layer.Lay all bread slices in a single layer into the dish.Whisk together the eggs, heavy cream, lemon zest, brown sugar, cinnamon, salt, vanilla, and rum (if using).Pour egg mixture over bread slices. Allow to soak for 10 min, flipping half way through.Heat 3 tbsp of butter and 1 tbsp of vegetable oil in large heavy skillet on medium heat.Fry bread slices for 2-3 min on each side, until golden brown.Place french toast slices in oven to keep warm while cooking remaining french toast. Wipe the skillet between batches adding more butter and oil as needed.* Top the french toast with your favorite topping, like syrups, fresh fruits, powdered sugar, jam.... Enjoy!