This pale green froth of a soup is the essence of fresh peas and a lovely opener to a meal. Plan to make it just before you serve it, unless you want to serve it chilled. Possible garnishes include truffle salt, minced tarragon, chervil, snipped chives, or a spoonful of cream.
FOR THE PEA STOCK: Bring 1 quart (946 ml) of water to a boil. As it's heating, add all but 1 1/2 C of the the scallions or leeks, the parsley, and the 1/2 tsp salt. Add about 3 cups of pea pods as you shell them if using English pod peas (reserve peas in a separate bowl), or whole pea pods if using mange-tout or other fresh peas. Once the water comes to a boil, lower the heat. Simmer for 20 minutes, then strain the liquid into a bowl. You may want to reserve a few whole peas in the pod to sliver for a garnish.
While the stock is cooking, wash and slice the leeks. (If you have already done so, proceed to step 3.)
Melt the butter in a soup pot and add the sliced leeks or scallions. Cook over medium heat for about a minute, stirring to lightly coat in butter, then add 1/2 cup of the stock so that the onions stew without browning. After 4 to 5 minutes, add the peas, another 1/4 teaspoons salt, and the sugar. Pour in 2 1/2 cups of the stock, bring to a boil, and simmer for 3 minutes or less. Meanwhile, prepare your garnishes (minced chives, a few slivered pea pods, and so on.)
Carefully (it's hot!) transfer the soup to a blender or use an immersion blender to fully liquify. If using a blender, drape a towel over the lid, and give a few short pulses to make sure it won't splatter. Then puree at high speed for 1 minute. Taste for seasoning. Pour into small soup bowls, garnish with chives or a few slivered peas in the pod, a few twists of fresh pepper, a spoonful of cream, or the like, and serve immediately.