Melt butter and oil in a large saucepan over medium heat. Add onion and 1/4 teaspoon salt and cook, stirring often, until golden, about 5 minutes. Stir in curry powder, ginger, lemon zest and potatoes and simmer, stirring occasionally, for 5 minutes. Stir in broth, coconut milk and asparagus. Bring to a simmer over medium heat, partially cover and continue to cook until the potatoes are tender, about 15 minutes.

Puree the soup with an immersion blender or a regular blender (in batches) until smooth. (Use caution when pureeing hot liquids.) Season with the remaining 1/4 teaspoon salt and pepper.

Whisk creme fraiche (or sour cream), lemon juice and scallion greens (or chives) in a small bowl and garnish with a swirl of it.

Note: Creme fraiche is a tangy, thick, rich cultured cream commonly used in French cooking. Find it in the dairy section of large supermarkets, usually near other specialty cheeses. Sour cream can be used as a substitute, or you can make your own lower-fat version by combining equal portions of reduced-fat sour cream and nonfat plain yogurt.