Heat oven to 200C/180C fan/gas 6.
Cut the squash into large cubes, about
4cm/1½in across, then toss in a large
roasting tin with half the olive oil. Roast
for 30 mins, turning once during cooking,
until golden and soft.
While the squash cooks, melt the
butter with the remaining oil in a large
saucepan, then add the onions, garlic and
¾ of the chilli. Cover and cook on a very
low heat for 15-20 mins until the onions
are completely soft.
Tip the squash into the pan, add the
stock and the crème fraîche, then whizz
with a stick blender until smooth. For
a really silky soup, put the soup into a
liquidiser and blitz it in batches. Return
to the pan, gently reheat, then season to
taste. Serve the soup in bowls with swirls
of crème fraîche and a scattering of the
remaining chopped chilli.