Place the split peas into a large bowl and cover with water. Discard any peas that float to the surface. Drain the remaining peas and rinse in a sieve with cold running water.
Heat the oil in a large saucepan over medium heat and cook the onion, carrot and celery for 10 minutes. Add the split peas, stock, bay leaves and parsley stalks and stir. Bring to the boil and add the hock. Reduce