Butter a large flat microwave-safe casserole dish (I used a Corning Ware dish about 10" square.) Place half the potatoes in the dish. Top with half the cheese and remaining potatoes. In a 4-cup microwavable measuring glass, combine milk, butter, salt, pepper, onion and garlic. Heat on HIGH for 2 to 3 minutes until butter is melted and milk is steaming hot (watch carefully nearing end of time to avoid boiling mixture.) Pour over potatoes in casserole dish. Cover casserole dish and cook on HIGH 10 to 12 minutes until potatoes are crisp and tender. Remove cover. Top casserole with remaining cheese. Cook on HIGH 1 minute uncovered to melt cheese. (Note: If casserole dish can withstand an oven broiler, skip the microwave in this step and brown under an oven broiler 3-4 minutes or until top surface just begins to brown. (I rimmed the edges and handles of my Corning Ware dish with a double layer of heavy-duty foil for this. Wish I had a butane food torch on hand instead!) VARIATION: Sprinkle 1 ½ cups ham cut in small pieces between the two layers of potatoes.