In a 10-inch skillet, melt the butter or heat oil over medium-low heat. Add the onion and saute slowly, stirring often, until limp but not brown, 6-7 minutes. Stir in 3 cups stock and let simmer, partially covered, about 15 minutes.
Pour the tomatoes with their juice into a food processor. Add the maple syrup or honey, and puree. Add the pumpkin, and buzz again. Strain the stock, and add the strained-out onions to the processor. Buzz again, and, if an extra smooth soup is desired, put through a power strainer.
Add the tomato-pumpkin puree to the stock. Season with the salt. Reheat, and serve very hot, garnished with red pepper puree if desired. Serves 8-10 as a first course.