Tear Swiss chard leaves from thick white stalks; discard the stalks and coarsely chop leaves.
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.
Serrano Chile Vinegar:
Using a paring knife, make a small slit in each chile and place in a pint-sized jar or bottle with a shaker top.
Bring vinegar and salt, to taste, to a boil and cook until salt has completely dissolved. Remove from heat and let cool slightly.
Pour vinegar over chiles, cover and let sit at room temperature for at least 2 days.