Heat oil in a large nonstick skillet over moderately high heat until hot but not smoking. Add chard stems and cook, stirring, until softened, about 3 minutes. Stir in leaves and garlic; cook, stirring, until leaves begin to soften, about 2 minutes. Remove from heat;stir in vinegar, nuts, and raisins; salt and pepper to taste. Transfer to a bowl and serve immediately. (Can be made ahead. Let cool to room temperature, cover and chill put to 2 days. Bring to room temperature before serving.) Makes 3 cups.