Heat olive oil in a large saucepan over medium heat. Add onion and cook until translucent, about 5 minutes. Add zucchini and cook another 2 minutes; then add chicken broth and 1 cup basil leaves. Reduce heat to a simmer and cook 20 minutes.
Purée the soup in batches in a blender. Pour the soup through a strainer into a bowl, using a ladle to push any solid bits through. Add 2 tbsp. crème fraîche and 1/4 tsp. chili powder. Season with salt to taste.
Divide soup among bowls and garnish each with some crème fraîche, a sprinkle of chili powder, and a few basil leaves.
The Twist: Dress It Up. We love the simplicity of this soothing, mild soup, but to make it a little fancier, skip garnishing the soup with crème fraîche, chili powder, and basil in step 3, and instead halve 1 roasted red bell pepper, removing seeds and stem. Slice half into ribbons. Put the other half in a blender and purée with 1/2 cup crème fraîche or sour cream. Strain into a bowl, then drizzle over soup. Top with bell pepper ribbons and toasted pine nuts.
Note: Nutritional analysis is per 1 1/2-cup serving.