For the dip: Heat the oil in a medium pan set over medium heat. Add the yellow onions and half of the red onions and saute, stirring occasionally, until caramelized, about 20 minutes. Add the sherry and wine and cook, stirring continually, until the liquid reduces completely, about 20 minutes more. Refrigerate until completely cool, 15 to 30 minutes.
Add the chilled onions, reserved red onions, sour cream, salt, pepper and scallions to a large bowl. Serve at room temperature.
For the chips: Slice the potatoes into chips on a mandoline, about 1/8-inch-thick.
Put the oil in a 3- to 5-gallon pot. Bring the oil to 325 degrees F. Fry chips, turning occasionally, until golden brown and all the little bubbles around the chips stop forming, about 15 minutes. (Those bubbles are an indicator of moisture. If you take them out before they're done, your chips will be soggy.)
Drain chips on paper towels and season with sea salt and black pepper. Serve with dip.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.
Recipe courtesy of Bub and Pop's, Washington, D.C.