In a large saucepan, melt the butter over low heat. Stir in the garlic and cook 1 minute. Whisk in the cornstarch, salt and pepper until smooth.
Gradually add the milk and bring to a boil.
Cook the sauce stirring constantly until thickened, approximately 2-3 minutes.
Remove from heat and stir in the potatoes.
Arrange 1/3 of the potatoes on the bottom of a 2 quart baking dish coated with cooking spray. Top with 1/3 of the parmesan cheese and 1/3 of the ham.
Repeat with the remaining ingredients ending with the rest of the cheese.
Cover and bake 30 minutes.
Uncover and bake an additional 30-45 minutes or until the potatoes are tender and the sauce is golden brown and bubbling.
Remove from the oven and allow to stand 5-10 minutes before serving.
Notes
Scalloped potatoes made be assembled in advanced and stored in the refrigerator, covered, until ready to bake. (up to 48 hours in advance.) Bake as directed, uncovered, and serve immediately.