Make the cheese sauce: melt the butter in a medium saucepan

Stir in the flour, salt and pepper. gradually add the milk, whisking over medium heat until the sauce is thickened and bubbly.

Add the garlic and 1 cup of the shredded cheese, stirring to melt. remove from burner and set aside.

Butter a 4-6 qt slow cooker. Layer half of the potatoes evenly on the bottom, and sprinkle the Parmesan cheese if using over the potatoes. Pour half of the cheese sauce over the layer spreading if needed to cover all the potatoes.

Repeat with the remaining potatoes and cheese sauce. Sprinkle the remaining 1/2 cup of shredded cheese over the top

Cover and cook on low for 6-7 hours or on high for 4 hours.