Put tomatillos, onion, and jalapeno in a small saucepan with water to cover. Bring to a boil and cook until tomatillos are softened and pale green, about 5 minutes. Drain, run under cold water to stop cooking, and drain well.
Halve jalapeno lengthwise. For a milder dip, scrape out ribs and seeds. Thickly slice jalapeno. Transfer to food processor along with tomatillo mixture and cilantro, and pulse until small chunks remain.
Halve avocados lengthwise and discard pits. Scoop out flesh and transfer to a bowl. Using a potato masher or fork, mash avocado along with salt until chunky. Fold in tomatillo mixture and serve.