Peel the boniato and the red potatoes and chop them into large pieces; place each in a separate container of water.
Fill two medium saucepans with water and bring to a boil over high heat. Add some salt and cook the boniato and the potatoes separately until fork tender.
In a medium sauté pan over medium-high heat, add the oil and cook the onion until caramelized. Deglaze pan with sherry vinegar and season with salt and freshly ground pepper.
In a medium saucepan over medium heat, reduce the heavy cream by half. Whisk in the roasted garlic and season with salt and pepper.
In a large stainless-steel mixing bowl, combine the potatoes and onions and add the cream a little at a time while mashing the potatoes. Season the mixture with salt and pepper. The mashed mixture can be kept in the stainless-steel bowl covered with aluminum foil over a pot of simmering water for 1 hour.
Note: To roast the garlic, put the heads of garlic in aluminum foil and roast at 400°F until soft, about 45 minutes.
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