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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry produce.
Dice potatoes into ½-inch pieces. Peel garlic. Place potatoes and whole garlic in a medium pot with enough salted water to cover by 2 inches. Bring to a boil and cook until tender, 15-20 minutes.
Reserve ½ cup potato cooking liquid, then drain and return potatoes and garlic to pot.
Keep covered off heat until ready to mash.