Tear Swiss chard leaves from thick white stalks; discard the stalks and coarsely chop
Heat a large skillet over medium-high heat. Add the bacon and cook until golden brown and fat has rendered. Remove bacon to a plate with a slotted spoon. Add the onions and cook in the rendered fat until soft. Add the garlic and cook for an additional minute. Add the Swiss chard
leaves, season with salt and pepper and stir to coat the leaves. Cook until the leaves have completely wilted. Add the bacon back to the pan. Add the Serrano Chile Vinegar and continue cooking for 2 minutes. Serve with additional vinegar on the side.