1.Heat 1 tbs of the peanut oil in a large pot over medium heat. Add the shallots, cover and cook until softened, about 5 minutes. Add the stock and potatoes and bring to a boil. Reduce the heat to low, season with salt to taste, and simmer until the potatoes are soft, about 30 minutes. 2.Pass the potato mixture through a food mill into a large bowl. Stir in the milk and season with cayenne to taste. Return to the pot and keep warm over low heat. 3.Heat the remaining 1/2 tbs oil in a medium-size skillet over high heat. Add the watercress and stir-fry until just wilted, about 2 minutes. Remove from teh heat, toss with the sesame oil, and add to the soup, stirring to combine. Serve hot, garnished with the sesame seeds.