Step #1 Mix all ingredients except the sour cream in a 31/2 to 6 quart slow cooker (aka crockpot).
Step #2 Cover up & cook on low heating 8 to 10 hrs (or on a high heat setting for 3 to 5 hrs) or until squash is tender.
Step #3 Place one third to one half of the mixture at a time in a mixer or food processor/blender, cover & mix on high speed until smooth.
Step #4 Return mixture to the slow cooker (aka crockpot).
Step #5 Stir in the sour cream.
Step #6 Cover up & cook on a low heat setting for 15 mins or until the soup is hot.
Step #7 Stir.
Step #8 Garnish each serving with a dollop of sour cream or unflavoured natural yoghurt (preferably Greek yoghurt), & swirl it into the soup with the tip of a knife.
Step #9 For crunch & colour, sprinkle with poppy seeds or sliced pecans.
Step #10 If serving chilled: garnish with thin slices of Granny Smith apples & a sprinkling of finely sliced crystalised ginger.
Step #11 Chef's Notes: I made this with butternut pumpkin rather than squash, simply because that's what was available.
Step #12 To simplify the preparation stage, I wrapped the two halves of the butternut pumpkin I'd bought in foil wrap & steamed them in a low oven for about an hr.
Step #13 The pumpkin was then easy to scoop out, & I sliced it roughly with a large spoon after I'd put it in the crockpot.
Step #14 I used three onions, peeled, & instead of apple sauce, I used 4 Granny Smith apples, & two leeks, roughly sliced, cored & roughly sliced.
Step #15 I also did the mixing with an immersion mixer.
Step #16 I simply removed half of the soup into a bowl, & mixed each half, then returned what I'd removed to the crockpot.
Step #17 It smelt delicious while it was cooking, & tasted great.