Char the vegetables. Heat a dry cast-iron skillet or comal over medium-high. Add 4 tomatillos, 2 jalapeños, 1/4 of onion, and 3 unpeeled garlic cloves. Cook, turning occasionally, until blackened in spots and softened — about 6 minutes for the garlic, 8–10 minutes for the chiles, and 10–12 minutes for the tomatillos and onions. Transfer to a plate as each is done.

Cool and peel. Let everything cool until you can handle it. Slip the papery skins off the garlic cloves and discard. Remove the stems of the chiles. You can also remove the seeds of the chiles if you'd like them less spicy. Leave the char on the tomatillos and chiles — that's where the smoky flavor lives.

Blend. Add the charred vegetables, 1 large avocado, 1/2 cup sour cream, 1 large bunch cilantro, lime juice from 1 limes, 2 teaspoons kosher salt and 1 cup water to a blender. Blend until smooth and creamy, scraping down the sides as needed.

Taste and adjust. Add more salt or a squeeze more lime to taste. The salsa should be thick enough to cling generously to a chip but still pourable over tacos.

Serve. Serve immediately or transfer to a container, press a piece of plastic wrap on the surface then cover with a lid and refrigerate for up to 5 days.