Mix butter, thyme, and lemon in small bowl. Season with salt and pepper. (Butter can be made 3 days ahead. Cover and chill. Bring to room temperature before using.)

Place potatoes in large pot. Add enough cold water to cover. Bring to boil. Cover partially and cook until potatoes are very tender, about 20 minutes. Drain. Return potatoes to same pot and mash. Heat half and half in small saucepan over medium heat until hot; stir into potatoes. Set aside 2 tablespoons thyme-lemon butter. Mix remaining butter into potatoes. Season to taste with salt and pepper. Transfer mashed potatoes to large bowl. Top with 2 tablespoons butter and serve.

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