How to Make It
Soak shallot slices in buttermilk at room temperature while preparing remaining ingredients.
Cook pancetta in a large straight-sided skillet over medium until crisp, about 15 minutes. Drain pancetta on paper towels. Add oil to skillet, and increase heat to medium-high. Add onions, and cook, stirring every 5 minutes, until caramelized and brown in color, about 45 minutes. (Add up to 1 cup water, 1/4 cup at a time, after 40 minutes, stirring to loosen browned bits from bottom of skillet, if onions begin to stick.)
Reduce heat to medium; add spinach, kale, garlic, and three-fourths of the cooked pancetta to skillet. Cook, stirring occasionally, until greens are wilted, about 5 minutes. Add butter, and cook, stirring occasionally, until butter is melted. Stir in 1/2 cup of the flour. Add milk, 1 cup at a time, stirring until smooth after each addition. Bring to simmer. Add Asiago, Gruyère, and fontina, and cook, stirring often, until melted, about 10 minutes. Stir in vinegar and 1 tablespoon of the salt. Keep warm over low heat.
Pour oil to a depth of 3 inches in a deep, straight-sided pot; heat over medium-high to 350°F. Combine cornstarch and remaining 1 1/2 cups flour and 1 teaspoon salt in a large bowl. Dredge soaked shallot slices in flour mixture, and carefully place in hot oil. Cook until lightly golden and crisp, about 2 minutes. Remove from oil, and drain on paper towels.
Transfer dip to a serving bowl, and top with crisp fried shallots and remaining pancetta. Serve with naan bread or pita chips.