How to Make It
In a large saucepan, bring the stock to a boil. Add the asparagus tips and cook just until bright green, about 2 minutes. Using a slotted spoon, transfer the tips to a plate. Add the onion to the saucepan. Cover and cook over moderate heat until softened, about 5 minutes. Add the asparagus stalks and cook until just tender, about 3 minutes.
Using a slotted spoon, transfer the onion and asparagus stalks to a blender. Add the ricotta and puree until smooth and creamy, adding a bit of the broth to loosen the puree. Return the puree to the saucepan; stir well and season with salt and white pepper. Add the reserved asparagus tips and simmer the soup until just heated through.
Meanwhile, in a mini chopper, pulse the pistachios, scallions, vinegar and oil to form a coarse paste. Season with salt and white pepper. Ladle the soup into shallow bowls and swirl in a bit of the scallion-pistachio pesto.