Tear the Swiss chard leaves from the thick white stalks. Discard the stalks, coarsely chop leaves and set aside.
Heat a large skillet over medium-high heat. Add the bacon and cook until golden brown and the fat has rendered. Remove the 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, season with salt and pepper and stir to coat the leaves in the fat. Cook until the leaves have completely wilted. Add the bacon back to the pan and toss to combine. Add the Serrano Chile Vinegar and continue cooking for 2 minutes. Serve with additional vinegar on the side.
Serrano Chile Vinegar
Bring the chiles, vinegar and a teaspoon of salt to a boil in a medium nonreactive saucepan. Remove from the heat, pour into a bowl, cover and let steep at room temperature. Cool in the refrigerator. Strain through a fine strainer into a bowl or glass jar.