In a deep pot, preheat the oil to 375 degrees F.
On each of 4 to 6 appetizer plates, place 4 assorted slices of tomato, and season lightly with salt and freshly ground pepper. Drizzle 2 tablespoons of vinegar over each plate of tomato slices and then place 1 slice of mozzarella on each tomato slice.
Toss the onions in the buttermilk and drain. Dredge in the flour, shaking off any excess. Carefully drop the onions into the hot oil and fry until golden brown, around 2 minutes. Drain on paper towels and season with salt immediately. Top the cheese with the fried onions and chiffonade of basil. Drizzle a tablespoon of basil oil on each plate.
This recipe was provided by professional chefs and has been scaled down from a bulk recipe provided by a restaurant. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.