You can make this ahead, but it may thicken as it sits. Reserve 1/2 cup of cooking liquid; just before serving, gradually stir it into the mixture until it's the desired consistency.
Place first 3 ingredients in a Dutch oven; cover with water. Bring to a boil. Reduce heat; simmer 15 minutes or until tender. Drain.
Stir in butter and remaining ingredients; mash mixture with a potato masher to desired consistency.
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