Accompaniment: Warm maple syrup
Cut banana into 1/4-inch thick slices. In a bowl, whisk together half-and-half, eggs, vanilla, cinnamon, and nutmeg. Cut bread slices horizontally almost in half, leaving 1 edge intact. Stuff each pocket with 4 to 6 banana slices (do not overstuff). In a baking dish just large enough to hold bread slices in one layer, arrange slices and pour custard over them. Soak slices, turning them over once or twice to allow bread to absorb all liquid, about 15 minutes.
In a 12-inch non-stick skillet melt butter over low heat until foam subsides and cook slices until golden, about 5 minutes on each side.
Dust French toast with confectioners' sugar and serve with syrup.