Ingredients

• 1 fresh or frozen ear of corn, shucked
• 4 cups plus 2 tablespoons water
• 1 garlic clove, smashed
• 1 1/2 teaspoons salt
• 1 1/4 cups chopped white onion
• 1 fresh serrano chile, stemmed and coarsely chopped (including seeds)
• 2 firm-ripe California avocados (1 to 1 1/4 lb total)
• 3 tablespoons fresh lime juice
• 1/4 cup crema or sour cream
• 1 cup coarsely chopped fresh cilantro
• 1/4 cup olive oil
• 1/2 teaspoon salt
• a 3/4-inch melon-ball cutter

Instructions

1. Roast corn on rack of gas burner over high heat, turning occasionally with tongs, until kernels are charred in spots, 4 to 5 minutes. (Alternatively, heat a dry well-seasoned cast-iron skillet and roast corn over moderately high heat, turning occasionally, about 10 minutes.) Transfer corn to a cutting board and, when cool enough to handle, cut kernels from cob with a sharp knife, then cut cob into thirds.
2. Bring kernels, cob pieces, 4 cups water, garlic, salt, and 1/2 cup onion to a boil in a 3-quart saucepan and boil until liquid is reduced to about 3 cups, about 20 minutes. Remove from heat and cool, uncovered. Discard cob pieces.
3. Purée corn mixture along with chile and remaining 3/4 cup onion in a blender, then pour through a fine-mesh sieve into a bowl, pressing hard on and then discarding solids. Return broth to cleaned blender.
4. Quarter, pit, and peel 1 avocado, then add to blender with 2 tablespoons lime juice and purée until smooth. Transfer soup to a bowl and cover surface with plastic wrap. Cover bowl with plastic wrap and chill soup at least 1 hour.
5. Purée cilantro, oil, and salt in cleaned blender, scraping down sides of blender several times. Pour oil into cleaned fine-mesh sieve set over a bowl and let drain 15 minutes (do not press on solids). Discard solids.
6. Halve and pit remaining avocado and scoop small balls from flesh with melon-ball cutter, then toss gently with remaining tablespoon lime juice in a bowl.
7. Whisk together crema and remaining 2 tablespoons water in a small bowl until smooth.
8. Season soup with salt and ladle into 6 shallow soup bowls. Divide avocado balls among bowls, then drizzle with crema and cilantro oil.