Preheat oven to 350°F.
Place croissant slices on a baking sheet and toast in oven about 5–10 minutes. Allow to cool.
Whisk together eggs, sugar, milk, sherry, vanilla extract, cinnamon, and nutmeg. Set aside.
Melt butter in a 10 1/2" cast iron pan over medium to low heat. Add brown sugar and corn syrup. Stir until blended and bubbling. Remove from heat.
Arrange sliced plums in a circular pattern in pan. Dot with pecan halves and set aside.
Place toasted croissant pieces evenly over plums. Pour egg/milk mixture over croissants, gently pressing on them to absorb liquid.
Bake for 35–45 minutes or until custard is set.
Remove from oven and cool in pan on rack for 5 minutes.
Place a platter on top of the pan and invert bread pudding onto the platter. Serve warm, topped with sauce.