Butter a 9x13-inch baking pan. Place baguette slices into the bottom of the prepared pan.
Combine half-and-half, milk, eggs, vanilla extract, cinnamon, and nutmeg in a bowl. Pour over bread slices, cover the baking pan, and store in the refrigerator, 8 hours to overnight.
Preheat the oven to 350 degrees F (175 degrees C).
Uncover pan. Combine brown sugar, pecans, margarine, and corn syrup in a bowl and spoon over bread mixture, making sure to touch each slice.
Bake in the preheated oven until a toothpick inserted into the center comes out mostly clean, about 40 minutes.