In a large skillet, cook the bacon over moderate heat until crisp, about 5 minutes. Drain the bacon on paper towels. Add the onions and chard stems to the skillet and cook over moderately high heat, stirring occasionally, until softened, about 5 minutes.
Add the chard leaves to the skillet and cook over moderate heat, stirring a few times, until the leaves are wilted, about 5 minutes. Stir in the vinegar and cook for1 minute. Stir in the almonds and the bacon. Season with salt and pepper and serve.