Peel and finely dice the onions. Peel and chop the ginger. Cut the chilli lengthwise, remove seeds, wash, clean and chop finely. Cut the ends off the lemon grass and remove the outer leaves.
Press lemongrass sticks flat. Wash pumpkin, halve, core and cut into small pieces.
Heat the oil in a wide saucepan. Fry onions, ginger, chilli and lemongrass in it. Stir in pumpkin and fry briefly. Add stock and coconut milk, bring to the boil, cover and simmer for about 10 minutes.
Roast peanuts in a pan without fat while turning. Remove, let cool briefly and chop roughly. Remove lemon grass from the soup. Puree the soup finely with a hand blender. Season to taste with salt, pepper, 1 pinch of sugar and lime juice.
Arrange the soup, sprinkle with peanuts. Garnish with chilli, lemongrass and coriander if desired.