Put the potatoes, garlic and thyme in a medium saucepan, cover with cold water by about an inch and season generously with salt. Bring to a simmer over medium heat and cook until the potatoes are very tender and a thin knife will easily pierce through the center of the potatoes, about 20 minutes. Drain and discard the thyme.
Heat the milk in a microwave-safe bowl or measuring cup in the microwave until hot, about 1 minute. Using a potato masher or large fork and adding the hot milk gradually, smash the potatoes and garlic into a chunky puree. Stir in the butter, horseradish, sour cream and bacon; season with salt and pepper.
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