Combine 1 cup brown sugar, 1/2 cup butter, and 2 tablespoons corn syrup in a small saucepan. Cook over medium heat, stirring constantly, until thickened.

Pour caramel mixture into a 9x13-inch baking dish; sprinkle with 1/2 cup pecans. Place 6 bread slices on top; sprinkle with remaining pecans and cover with remaining 6 bread slices.

Combine eggs, milk, cinnamon, vanilla, nutmeg, and salt in a blender; blend until smooth. Pour egg mixture evenly over bread slices. Cover baking dish with plastic wrap; refrigerate 8 hours or overnight.

Remove French toast from refrigerator 30 minutes prior to baking. Preheat the oven to 350 degrees F (175 degrees C).

Bake in the preheated oven until the French toast is puffed, lightly browned, and a knife inserted in the center comes out clean, 40 to 45 minutes.

Meanwhile, make the caramel sauce: Combine remaining 1/2 cup brown sugar, remaining 1/4 cup butter and remaining 1 tablespoon corn syrup; cook until thickened, stirring constantly.

Drizzle warm sauce over French toast just before serving.