MELT BUTTER IN A 2-QUART POT over medium heat. Add the onion and cook, stirring occasionally, until onion softens, about 10 minutes. Add the pumpkin and salt and continue to cook, stirring, another 15 minutes for fresh pumpkin, 5 minutes for canned. Add the stock, salt, coriander, curry and pepper. Cover, increase heat to high and bring to a boil. Reduce heat to low and simmer 15 minutes. Remove from heat and place in a food processor or blender and puree until smooth. Combine puree, milk and walnut oil. Cover, bring to the boil and cook 2 minutes. Serve piping hot.