Cold Avocado Corn Soup With Cilantro Oil

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. 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. 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. 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.