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.
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.
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.
In a food processor, pulse vegetables and any juices; avocado, if using; cilantro; and garlic until coarsely puréed. Scrape into a bowl and stir in 1/4 cup water, lime juice, and 3/4 tsp. salt. Season to taste with salt.
*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.
Make ahead: Chill up to 2 days; if using avocado, smooth plastic wrap against surface and chill up to 1 day only.
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.
Note: Nutritional analysis is per 1/4-cup serving.