Cheesy Scalloped Potatoes

Bahan-bahan (6 porsi)

• 2 pounds yellow potatoes
• 10.5 ounces condensed cream of mushroom soup (chicken or celery soup, 1 can)
• 1 cup milk
• ½ teaspoon garlic powder
• black pepper (to taste)
• 1 small onion (thinly sliced)
• 2 cups shredded sharp cheddar cheese