In heavy-bottomed sauce-pot, heat butter and oil on medium-low heat. Add onions and cook 10 minutes or until translucent, stirring occasionally. Add butternut squash, stock, pumpkin, 2 teaspoons salt, and 1/2 teaspoon pepper; heat to boiling on medium-high heat. Cover and simmer on medium-low heat for 20 minutes or until squash is very tender, stirring occasionally. Process mixture with hand blender until smooth (or put through food mill and return to pot). Add half-and-half to soup; heat slowly. Serve with garnishes, if using.