1) Fill a large bowl with cold water. Trim and discard the last 5cm of the chard stems. Add leaves to water. If the greens are very dirty, first wash them in several changes of water. Lift the greens from the water, and put them in a colander to drain. Roughly chop the greens.
2) In a large casserole dish or tall-sided skillet over medium heat, cook the onion, garlic, red pepper flakes, cider vinegar and nutmeg for about 2 minutes.
3) Add the greens in batches, stirring each batch as they wilt, before adding more. Add the chicken stock and cook, covered, stirring once or twice, until tender, about 10 minutes. Adjust the seasoning with vinegar, salt and pepper, to taste.