In a large Dutch oven or heavy pot over medium-low heat, heat the olive oil. Add the onions, carrots and celery, seasoning lightly with salt and pepper. Cook until the onions are translucent, then stir in the chard and farro. When the chard begins to wilt, pour in the chicken stock and bring to a boil. Lower heat to a simmer, and let it cook for about 20 minutes, or until the farro has puffed up and expanded. Season to taste with salt and pepper. Stir in the sausage and the Sherry vinegar and adjust seasoning once more if necessary. Serve hot, with a drizzle of fresh olive oil and crusty baked bread.