In a 4-quart Dutch oven, melt butter. Add leeks and garlic. Cook and stir over medium heat for 5 minutes. Sprinkle in cumin and stir for 30 seconds. Add sweet potatoes, broth, salt and black pepper. Bring to boiling; reduce heat. Simmer, covered, for 15 to 20 minutes or until potatoes are tender.
Stir pumpkin, corn, celery soup, milk and thyme into potato mixture. Cook over medium heat until heated through, stirring occasionally. To serve, sprinkle soup with pumpkin seeds. If you like, garnish soup with sour cream and serve with crackers. Makes 6 servings (8 cups).