In a large saucepan of boiling salted water cook the chard ribs for 7 minutes, add the leaves and cook for an additional 3 minutes. Drain the chard, rinse it under cold water to stop the cooking, and pat it dry between paper towels. Finely chop the chard. (The chard may be prepared 2 days in advance and kept chilled in a zip lock plastic bag.).
In a heavy skillet cook the onion in 1/3 cup of the oil over moderately low heat, stirring until it is softened, add the chard, the parsley, and salt and pepper to taste, and cook the mixture, stirring for 5 minutes. Stir in the tuna and cook the mixture until the tuna has just heated through.
In a kettle of salted boiling water cook the spaghetti until it is al dente, drain it well, and in a large bowl toss it with the tuna mixture, the wine vinegar and salt and pepper to taste. Drizzle additional olive oil over the pasta just prior to service.
Serves 12 as part of an Italian Christmas Eve Buffet Dinner.