Put 1/4 cup of the broth and the leeks, onion, celery, shallots, and garlic in a large pot and over medium heat. Cook, stirring ocasionally, for about 5 minutes. Do not brown the vegetables. Stir in the sunchokes and the remaining 1 3/4 cups of broth. Increase the heat to medium-high and bring to a simmer. Stir in the soymilk and return to a simmer. Cook for 20 minutes, using a ladle to skim off any foam that forms at the surface. Discard the foam. Transfer the soup to a blender and process on high speed until smooth. You may need to process the soup in batches depending on the size of your blender. Serve immediately, topped with the croutons and watercress. Note: Sunchokes are also known as jerusalem artichokes. Because they turn brown quickly after they are peeled, it's best to cook them as soon as possible.