In a large saucepan, saute leeks in butter until tender. Stir in the squash, broth, thyme and pepper. Bring to a boil. Reduce heat; cover and simmer until squash is tender, 10-15 minutes . Cool slightly.
In a blender, cover and process squash mixture in small batches until smooth; return all to the pan. Bring to a boil. Reduce heat to low. Add cheese; stir until soup is heated through and cheese is melted. Garnish with sour cream and onion.
