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
• Special equipment: 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. Prepare cilantro oil while soup chills: 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. 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.
7. Assemble soup: 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. Whisk together crema and remaining 2 tablespoons water in a small bowl until smooth. Season soup with salt and ladle into 6 shallow soup bowls. Divide avocado balls among bowls, then drizzle with crema and cilantro oil.
8. 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.
9. Whisk together crema and remaining 2 tablespoons water in a small bowl until smooth.
10. Season soup with salt and ladle into 6 shallow soup bowls. Divide avocado balls among bowls, then drizzle with crema and cilantro oil.