1. Heat the oil in a large heavy based saucepan or casserole. Add the onions, celery and carrot, sweating them for about 10 minutes until just beginning to soften. Add the split peas, mixing well, and cook for a further couple of moments.
2. Pour in the stock, cover and bring to a boil. Reduce to a low temperature then simmer for about 30 minutes or until all the vegetables are soft.