1. Heat the butter in a heavy saucepan over medium heat. Add the onions and sauté until wilted, about 5-6 minutes. Stir in the curry powder, cook for 2 minutes, then add the potatoes, stock and water. Bring the liquid to a boil, then reduce the heat and simmer until the potatoes are almost tender, about 10 minutes.
2. Slice the asparagus crosswise into ½-inch pieces and add them to the saucepan. Partially cover and simmer until the asparagus and potatoes are completely tender. Cool slightly and transfer to the jar of an electric blender, pour in the chicken broth and water and purée until the soup is completely smooth, then return to pot and season with salt.
3. Blanch the reserved asparagus tips in boiling salted water until tender; drain refresh. Heat the soup until hot. Add in the cream and season to taste with salt and lemon juice. Stir in the lemon zest and serve, garnishing with the asparagus tips.