Melt butter over medium-low heat in a large stockpot or Dutch oven. Cook onion, celery and carrots with salt and pepper until onion is soft, about 7 to 10 minutes. Add the turmeric, stock, peas, potatoes, and ham hock, and simmer, covered, about 1 hour. Remove the ham hock. Puree soup in a blender or food processor until smooth. Return to clean pot, and adjust the seasonings. Serve immediately in flat soup dishes with wide (3-inch) rims, garnished with dark bread croutons. Place brown mustard on the rims of the bowls, and take a little mustard with each spoonful of soup.
c.1997, M.S. Milliken & S. Feniger, all rights reserved