Preparation Julienne skin (only) from half of zucchini with slicer; toss with 1/2 teaspoon salt and drain in a sieve until wilted, at least 20 minutes. Coarsely chop remaining zucchini. Cook onion and garlic in oil in a 3- to 4-quarts heavy saucepan over medium-low heat, stirring occasionally, until softened, about 5 minutes. Add chopped zucchini and 1 teaspoon salt and cook, stirring occasionally, 5 minutes. Add 3 cups water and simmer, partially covered, until tender, about 15 minutes. Purée soup with basil in 2 batches in a blender (use caution when blending hot liquids). Bring remaining cup water to a boil in a small saucepan and blanch julienned zucchini 1 minute. Drain in a sieve set over a bowl (use liquid to thin soup if necessary). Season soup with salt and pepper. Serve in shallow bowls with julienned zucchini mounded on top.
Preparation Julienne skin (only) from half of zucchini with slicer; toss with 1/2 teaspoon salt and drain in a sieve until wilted, at least 20 minutes. Coarsely chop remaining zucchini. Cook onion and garlic in oil in a 3- to 4-quarts heavy saucepan over medium-low heat, stirring occasionally, until softened, about 5 minutes. Add chopped zucchini and 1 teaspoon salt and cook, stirring occasionally, 5 minutes. Add 3 cups water and simmer, partially covered, until tender, about 15 minutes. Purée soup with basil in 2 batches in a blender (use caution when blending hot liquids). Bring remaining cup water to a boil in a small saucepan and blanch julienned zucchini 1 minute. Drain in a sieve set over a bowl (use liquid to thin soup if necessary). Season soup with salt and pepper. Serve in shallow bowls with julienned zucchini mounded on top.