In a small saucepan, melt butter. Stir in the flour, salt, pepper and Cajun seasoning until smooth. gradually add milk, sherry and Worcestershire sauce. Bring to a boil; cook and stir for 1 minute or until thickened. Reduce heat; stir in 1/4 cup cheese. Remove from the heat; set aside.
Place half of the potato slices in a 1-qt. baking dish coated with nonstick cooking spray. Layer with ham, onion and half of the white sauce. Repeat layers. Cover and bake at 350° for 50-60 minutes or until potatoes are tender. Uncover; sprinkle with remaining cheese. Bake 5-10 minutes longer or until cheese is melted.