Baked Mashed Potatoes and Yams with Garlic and Parmesan

Preparation Preheat oven to 350°F. Place garlic in small ovenproof cup or ramekin. Drizzle 2 tablespoons olive oil over. Bake until garlic is very soft, about 25 minutes. Cool; peel garlic, reserving oil in cup. Brush 8x8x2-inch glass baking dish with 1/2 tablespoon olive oil. Cook all potatoes in large pot of boiling salted water until tender, about 25 minutes. Drain, reserving 1 cup cooking liquid. Return all potatoes to same pot. Add milk, butter, rosemary, roasted garlic and reserved oil to potatoes. Mash until smooth and fluffy, adding enough reserved cooking liquid to thin to desired consistency. Mix in 3 tablespoons cheese. Season with salt and pepper. Spoon potatoes into prepared dish. Sprinkle 3 tablespoons cheese over; drizzle with 1 tablespoon oil. (Can be made 2 hours ahead. Cover; let stand at room temperature). Preheat oven to 350°F. Bake potatoes uncovered until heated through and golden on top, about 45 minutes.