For the filling: In a medium (3 to 4-quart) saucepan, bring about 1 quart of water to a boil. Add the cubed potatoes and heavily salt the water (about 2 teaspoons), Simmer until the potatoes are fully tender, about 10 minutes. Drain.
In a large (12-inch) heavy well-seasoned or nonstick skillet or griddle set over medium heat, combine the chorizo and onion; stir regularly, breaking up any clumps of sausage, until the onion is soft and the sausage cooked through, about 10 minutes. If the sausage has rendered more than a light coating of fat over the bottom of the skillet, pour out the excess.
Add the potatoes to the skillet and continue to cook over medium heat, stirring regularly, until the potatoes begin to brown, about 8 minutes. As the mixture cooks, mash everything together a little with the back of a spoon or spatula, scraping up any crusty bits of potato, so that it roughly holds together (it'll look a little like hash). Cover and keep warm over the lowest heat.
For the salsa: As the filling finishes cooking, make the salsa: In a food processor, combine the tomatillos, garlic and chiles. Pulse the machine until everything is finely chopped. Peel and pit the avocado, add the flesh to the processor and pulse until everything is well blended (the salsa will be the consistency of a light mayonnaise). Scrape the salsa into a serving bowl, taste and season with salt, usually about 1/2 teaspoon.
Serving: Scrape the warm chorizo filling into a serving bowl and set on the table along with the avocado salsa and a basket of steaming tortillas.
Note: If you are using store-bought tortillas, drizzle a clean kitchen towel with 3 tablespoons water and wrap your cold tortillas in it. Slide the package into a microwaveable plastic bag and fold the top over, but don't seal it. Microwave at 50% power for 4 minutes to create steam in the package. Let stand for 2 or 3 minutes before serving.