Mix all ingredients except the sour cream in a 31/2 to 6 quart slow cooker.
Cover and cook on low heating 8 to 10 hours (or on a high heat setting for 3 to 5 hours) or until squash is tender.
Place one third to one half of the mixture at a time in a blender or food processor, cover and blend on high speed until smooth. Return mixture to the slow cooker.
Stir in the sour cream. Cover and cook on a low heat setting for 15 minutes or until the soup is hot. Stir.
Garnish each serving with a dollop of sour cream or unflavoured natural yoghurt (preferably Greek yoghurt), and swirl it into the soup with the tip of a knife.
For crunch and colour, sprinkle with poppy seeds or chopped pecans.
If serving chilled: garnish with thin slices of Granny Smith apples and a sprinkling of finely chopped crystalised ginger.
Chef's Notes: I made this with butternut pumpkin rather than squash, simply because that's what was available. To simplify the preparation stage, I wrapped the two halves of the butternut pumpkin I'd bought in foil and steamed them in a low oven for about an hour. The pumpkin was then easy to scoop out, and I chopped it roughly with a large spoon after I'd put it in the crockpot. I used three onions, roughly chopped, and two leeks, and instead of apple sauce, I used 4 Granny Smith apples, peeled, cored and roughly chopped. I also did the blending with an immersion blender. I simply removed half of the soup into a bowl, and blended each half, then returned what I'd removed to the crockpot. It smelt delicious while it was cooking, and tasted great.