In a medium saucepan, cover 1/2 of the potatoes with cold water and a pinch of salt. Bring to a boil and blanch until tender, about 5 minutes. Remove from heat, drain and reserve.
In a large pot over medium heat, melt the butter. Add the onions and leeks and stir. Cover the pot and saute the onions and leeks until they are translucent, stirring occasionally, for 5 minutes. Uncover, and add the raw potatoes, the bouquet garni, salt and pepper, chicken stock and cream, and stir. Bring the soup to a simmer over medium heat, uncovered. Simmer until the potatoes are soft, cooking for about 15 minutes. Remove from heat.
Using a blender, puree the soup in batches. Taste for seasoning and adjust with salt and pepper. Return the soup to the pot, add the blanched potatoes, and place over medium heat. When the soup begins to simmer, add the watercress and scallops, and cook for 30 seconds, stirring once. Ladle the soup into hot bowls and sprinkle with chives. Serve immediately.