IN A STOCK POT, over low heat, melt the butter and add the onion, carrot, celery and garlic. Let the vegetables cook, stirring, until limp, about 15 minutes, being careful not to brown. Add the split peas, stock, ham hock and lemon and bring to a boil. Reduce the heat to low and simmer for 30 minutes, or until the peas are completely soft. Remove the ham hock and the lemon. Add the sorrel and cream. Transfer to a food processor or blender and puree until smooth. Strain through a fine sieve. Taste for salt and pepper and adjust as desired. Serve this soup piping hot or well chilled.
NOTE: If the soup is chilled, thin it with an additional cup of stock, low-sodium broth or water.