Peel the pumpkin, cut into slices and scrape out the inner marrow with a spoon. Cut the pumpkin into small cubes. Peel potatoes, cut in half. Onions peel and halve.
Bring a litre of salt water with bay leaf and onions to the boil. Cook the prepared vegetables for 20 minutes. Meanwhile wash the crabs. Wash the chives and cut into small rolls. Scoop half the vegetables out of the stock.
Remove the bay leaf. Purée the rest in the liquid. Stir in remaining vegetables, crabs, chives and peppercorns. Refine with cream. Bring to the boil again briefly and season to taste with salt, ground chilli and vinegar.