Mash the onion, chile, salt (the coarseness of kosher salt helps you make the paste), and half of the cilantro to a paste in a molcajete or other mortar. You can also mince and mash the ingredients together on a cutting board with a large knife or a fork, and then transfer the paste to a bowl.
Score the flesh in the avocado halves in a crosshatch pattern (not through the skin) with a knife and then scoop it with a spoon into the mortar or bowl. Toss well (it should be like salad properly dressed in vinaigrette), then add the rest of the cilantro and mash very coarsely with a pestle or a fork. Season to taste with lime juice (if you’d like) and additional chile and salt.