Cook the potatoes. Peel the potatoes if you like, then cut them into ¼-inch pieces and scoop them into a microwave-safe bowl. Splash on about 3 tablespoons water and sprinkle generously with salt. Cover with a vented cover or plastic wrap with a few poked holes and microwave for 4 minutes at 100% power—they should be tender. Immediately tip off the water (it’ll be steamy hot!) and scoop out the potatoes onto paper towels. Pat dry. This is essential for good browning.
Brown the chorizo and onions. Scoop the chorizo into a large (10-inch, preferably non-stick or seasoned cast iron) skillet set over medium heat. Break it into smaller pieces with a spatula as the fat begins to render out. (If little renders, add a little oil, drippings or lard.) Add the onions and cook, stirring regularly, until the chorizo is cooked through and the onions are golden, 8 to 10 minutes. At this point you need a couple of tablespoons of visible fat in the pan; add more if necessary.
Brown the potatoes and serve. Turn up the heat to medium-high. Stir the potatoes into the mixture, coating them with the fat. Cook, stirring every once in a while, until the potatoes are browned, 5 to 8 minutes longer. Taste and season with salt if you think necessary. Serve with warm tortillas, tomatillo salsa and avocado for making delicious tacos.