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1
Preheat broiler and set a rack 3 in. from heating element. Line a rimmed baking pan with foil and set tomatillos, onion, poblano, and serranos in it.
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2
Broil the vegetables, turning as needed, until tomatillos and onion are speckled brown and chiles are black all over, 12 to 15 minutes; as vegetables are done, transfer to a bowl. Cover vegetables with a plate or foil and let stand about 5 minutes for chile skins to loosen.
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3
Pull off stems and blackened skins from chiles; for best flavor, don't rinse chiles (a few blackened bits are okay to leave on). Open poblano and remove seeds.
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4
In a food processor, pulse vegetables and any juices; avocado, if using; cilantro; and garlic until coarsely pureed. Scrape into a bowl and stir in 1/4 cup water, lime juice, and 3/4 tsp. salt. Season to taste with salt.
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*Tart-tasting tomatillos look like green tomatoes with papery husks. Poblanos (sometimes mislabeled as pasillas) are large, meaty, deep green chiles with a fairly mild flavor; find them in your grocery store's produce section.
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6
Make ahead: Chill up to 2 days; if using avocado, smooth plastic wrap against surface and chill up to 1 day only.
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7
Add Heat to Taste: You can control the heat of a salsa by adjusting the heat of the chiles. Slice off the top of each chile, being sure to cut through the ribs and seeds, where the heat-producing compound capsaicin is concentrated. Test the chile's fire by touching the top to your tongue (each chile has a different heat level). Adjust the heat, if you want a milder salsa, by splitting the chile and scraping out some or all of the ribs and seeds. If your skin is sensitive, wear kitchen gloves or hold chiles with a fork-and don't touch your eyes.
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8
Note: Nutritional analysis is per 1/4-cup serving.
