Zucchini soup with crème fraîche

Olive oil, extra virgin	2 tablespoons
Onions, medium, coarsely chopped	2
Zucchini, medium, quartered lengthwise, cut crosswise ½-inch thick	1 ¾ pounds
Vegetable stock	3 cups
Salt
Pepper, freshly ground
Crème fraîche	3 tablespoons
White wine or sherry vinegar	Drizzling

1. In a saucepan over moderate heat, warm the oil, and then add the onions; cook stirring occasionally until soft, about 5 minutes. 
2. Add zucchini, and cook, stirring, until sizzling, about 2 minutes. 
3. Add the stock, and bring to a boil. 
4. Simmer over moderate heat until the zucchini is tender, about 10 minutes. 
5. Working in batches, puree the soup in a blender. 
6. Rewarm in saucepan, season with salt and pepper, and whisk in crème fraîche.
7. Ladle into bowls, and drizzle with sherry vinegar.