1 pound pencil-thin asparagus, tips reserved and stalks cut into 1-inch
1/2 small onion, thinly sliced
1 pound fresh ricotta (2 cups)
Salt and freshly ground white pepper
1/4 cup salted roasted pistachios
2 scallions, cut into 1/2-inch lengths
1 teaspoon white wine vinegar
1/4 cup extra-virgin olive oil
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.
The soup and pesto can be refrigerated separately for up to 2 days.