2 teaspoons butter
1 medium onion (about 5 1/2 ounces), thinly sliced
3 garlic cloves, minced
4 cups 2% reduced-fat milk
1 teaspoon salt
1/4 teaspoon freshly ground black pepper
3 ounces all-purpose flour (about 2/3 cup)
6 ounces shredded sharp cheddar cheese (1 1/2 cups), divided
6 ounces diced ham (about 1 1/4 cups)
3 pounds peeled baking potatoes, cut into 1/8-inch-thick slices
Cooking spray