Tear Swiss chard leaves from thick white stalks, discard the stalks and coarsely chop leaves, set aside.
Heat a large skillet over medium-high heat. Add the bacon and cook until golden brown and fat as 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, 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. Add the vinegar and continue cooking for 2 minutes. Serve with additional vinegar on the side.
Serrano Chile Vinegar:
4 serrano chiles, coarsely chopped
2 cups white wine vinegar
Bring chiles, vinegar, and a teaspoon of salt to a boil in a medium non-reactive saucepan. Remove from heat, pour into a bowl, cover and let steep for at least 2 hours or overnight at room temperature. Strain through a fine strainer into a bowl or glass jar.