Instructions: For the potatoes, place in a medium pot and cover with cold salted water. Bring to a boil over medium-high heat. Reduce heat and simmer until very tender and almost falling apart, for about 20 minutes; drain.

Set a large holed sieve over the same pot. Mash the potatoes through the sieve, using a large mallet and up/down motion. Stir in the vinegar. Season with salt and pepper. (This can prepared ahead, covered and set aside at room temperature.)

Meanwhile, cover the beets with cold salted water in medium pot. Bring to a boil over medium-high heat. Reduce heat and simmer until very tender and easily pierced with a knife, for about 20 minutes; drain.

Puree the beets with the milk in a blender until smooth, pulsing on/off and stopping to scrape down the sides of the container. Season with salt and pepper. Strain the beet puree into the potatoes through a fine sieve, pressing on the beets. (This can be prepared ahead, covered and set aside at room temperature.)

To serve, stir the puree over medium-high heat until warm, using a wooden spatula. Add the butter and stir until smooth. (This can prepared 1 hour ahead to this point and kept warm in a larger pan of gently simmering water (bain marie). Stir occasionally.

Mix in the chives. Mound in a vegetable dish or spoon out onto 4 to 6 large plates with an entree. Serve immediately.

This recipe yields 4 to 6 servings.

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