The Stock : Melt the butter in a large saucepan over medium high heat. When hot, add the prawn shells and leeks and saute until the shells turn pink and the leeks are translucent. Add the thyme, parsley, water and wine and bring to a boil. Reduce the heat and simmer for 20 minutes, pressing down on the prawn shells to extract their flavor. Strain the stock through a fine sieve, again pressing on the prawn shells. Reserve the stock and discard the solids.
The Soup: In the same saucepan in which you Sauteed the prawns, add the onions and saute until translucent. Add the squash and rosemary, reduce heat to low, cover and cook, stirring occasionally, for 10 minutes. Add 5 cups of the stock and lemon juice. Bring to a boil, reduce heat to low, cover and cook until the squask is very tender, 1015 minutes. Discard the rosemary. Puree the soup in a food processor or food mill in small batches. Add the remaining 1 cup stock if the soup is you desire a thinner soup. Return the puree to the pan over medium heat.
Slice the prawns in half lengthwise. Add the prawns to the soup and cook until they turn pink. Transfer prawns to a warm platter and cover. Season soup to taste with salt and pepper and a few dashes of Tabasco.
To serve: Ladle the soup into shallow soup bowls and set 6 prawns on top of each. Garnish with rosemary