Asparagus Soup with Lemon-Herb Crème Fraîche

1/2 cup crème fraîche
1 tablespoon chopped parsley
2 teaspoons chopped chives
1 teaspoon chopped tarragon
1 teaspoon finely grated lemon zest
1 teaspoon fresh lemon juice
Kosher salt and freshly ground pepper
2 pounds medium asparagus, tough ends discarded, tips reserved and stalks coarsely chopped
2 tablespoons unsalted butter
1 medium onion, finely chopped
1 quart chicken stock
2 cups water

In a bowl, blend the crème fraîche with the parsley, chives, tarragon and the lemon zest and juice. Season with salt and pepper, then cover and refrigerate.
In a small saucepan of boiling salted water, cook the asparagus tips until tender; drain and transfer to a plate.
Melt the butter in a large saucepan. Add the onion and cook over moderate heat, stirring, until softened, 4 minutes. Add the stock and water and bring to a boil. Add the chopped asparagus and season with 1 1/2 teaspoons of salt and a few grindings of pepper. Boil over moderately high heat until the asparagus are tender, about 10 minutes. Remove from the heat and let stand for 5 minutes.
Working in batches, puree the soup in a blender and pour it into a clean saucepan. Season with salt and pepper. Stir in the asparagus tips and bring to a simmer over moderate heat. Ladle the soup into warmed shallow bowls. Top with large dollops of the lemon-herb crème fraîche and serve.