Lightly butter a 9- by 13-inch pan. Lay baguette slices on a cut side in a single layer in pan, squeezing if needed to make them fit. In a blender or in a bowl with a whisk, whirl or beat eggs, half-and-half, milk, nutmeg, cinnamon, and vanilla to blend well. Pour egg mixture over bread to moisten evenly. Cover and refrigerate at least 8 or up to 24 hours.
In a 1- to 2-quart pan over medium heat, stir 1/4 cup butter with brown sugar, corn syrup, and pecans until butter melts. Uncover bread mixture and spread slices evenly with the butter mixture, using all of it.
Bake, uncovered, in a 350° oven until topping is browned and bubbly, about 45 minutes. Scoop portions into bowls.