Asparagus Soup with Lemon Crème Fraîche

Ingredients

• 3 pounds asparagus, ends removed and discarded
• 2 tablespoons unsalted butter
• 1 onion, coarsely chopped
• 6 cups chicken stock
• 2 tablespoons lemon juice
• Salt and freshly ground black pepper
• 1/2 cup crème fraîche
• 1 teaspoon grated lemon zest
• 1 to 2 tablespoons water