Bring beef broth to a boil and add potatoes, carrots, tomatoes, leeks, celery root, parsley, bay leaf, thyme, salt and pepper. Return to boil, reduce heat, cover, and simmer for 30 minutes. Meanwhile, dice the pumpernickel and the bacon. Fry bacon in skillet until partly cooked, the add pumpernickel cubes and fry until bacon is dark and bread is crisp. Set aside. When vegetables are tender remove parsley and bay leaf and blend in sour cream or yogurt. Adjust seasonings to taste. Simmer for another minute. Ladle chowder into serving bowls and garnish with croutons and bacon. The flavor improves with reheating, so make chowder in advance if possible. Makes 4 to 6 servings. This recipe, by Heidi M. Smith, won second prize for potatoes in the 2nd Annual Great New England Cookoff, sponsored by Yankee Magazine. Christie Aspegren, September 93 Round Robin.