Cut the pumpkin into large chunks about 4cm in size removing the seeds and pith; no need to peel as the skin will soften during cooking. Toss in a large roasting tin with half the oil and roast for 30 minutes turning once during cooking. Cook until golden and soft.
As the pumpkin is cooking melt the butter and remaining oil in a big saucepan. Add the onions, garlic and 3/4 of the chilli. Cover and cook over a very low heat for 15-20 minutes until the onions are completely soft.
Tip the squash into the saucepan then add the stock. Blend with a hand held blender until smooth.
Serve with a scattering of the remaining diced chilli and a dollop of creme fraiche if desired.