Cook and shred the meat. Place the meat in a medium (2- to 3-quart) saucepan and cover with 4 cups of water. Add the herbs and 2 teaspoons salt. Slice the top quarter off the onion, add it to the pan, bring to a boil, partially cover and simmer of medium to medium-low heat for 1 ½ to 2 hours, until the meat is thoroughly tender. Remove and cool to room temperature. Pull into thin strings—nothing thicker than kitchen twine or about 1/16th of an inch. Fair warning: this takes some patience.
Fry the onion and meat. Heat the oil over medium to 325 degrees (temperature is important here; too hot and the onions will burn). Cut the remaining onion in half through the root end, then thinly slice each half—about 1/8th inch is great. Scoop into a bowl and toss with the flour and ½ teaspoon salt. Scoop up half the onions, gently shake them to release excess flour, then slowly release them into the hot oil. Stir them regularly to keep the pieces separate, until they are richly golden, about 2 minutes. Use a spider/strainer or slotted spoon to scoop onions onto paper towels to drain. Sprinkle with salt, then repeat with the remaining onions.
Raise the heat below the oil to medium-high. When it reaches 360 degrees, sprinkle in about ½ cup of meat and stir to keep the strands separate. When crispy—about 2 minutes—use the spider/skimmer to remove the crispy strands to drain on paper towels. Sprinkle with salt while warm. Repeat with the remaining meat.
Make the simple guacamole. Place the garlic in a small, microwave-safe dish, cover with water and microwave for 1 minute to soften and transform the raw garlic flavor. Drain. Roughly chop, then mash to a rough paste with the side of your knife. Cut around the avocados, remove pits, then scoop the flesh into a small bowl. Add the garlic, oregano and lime. Mash everything together with an old-fashion potato masher or back of a spoon. Taste and season with salt, usually a generous teaspoon.
Make tacos. As you’re ready to serve, build tacos: On a warm tortilla, spread a portion of guacamole in the middle, splash with salsa, then top with a pile of crispy beef and onions. A sprinkling of cilantro completes the picture.