• Give spring onion, vinegar, and a pinch of salt a good toss in a small bowl. Let sit while you prepare the hash; this will be just enough time to lightly pickle the onion.
  
  
• Heat 1 Tbsp. oil in a medium skillet over medium. Cook chorizo, breaking up with a wooden spoon and stirring occasionally, until brown and cooked through, about 5 minutes. Transfer chorizo to a plate with a slotted spoon.
  
  
• Add yellow onion and garlic to fat in skillet and cook, stirring occasionally, until softened, 6–8 minutes.
  
  
• Meanwhile, grate potato on the coarse holes of a box grater; squeeze firmly over sink to remove excess water.
  
  
• Add potato, chorizo, and remaining 1 Tbsp. oil to skillet. Cook, turning hash occasionally and breaking up with a spoon, until potato is golden brown, cooked through, and crunchy around the edges, 8–10 minutes.
  
  
• Assemble tacos with hash, fried eggs, tortillas, watercress, drained pickled spring onions, and cotija. Serve with lime wedges for squeezing over.
  
  

Olive Oil Eggs

• Heat oil in a medium nonstick skillet (a 10" skillet is perfect for 2 eggs) over medium-high. When oil is shimmering, carefully add eggs one at a time, shaking pan gently after each addition. Cook eggs, shaking pan occasionally to keep them from sticking to each other (a rubber spatula can help nudge them apart if needed), until edges are golden brown, about 2 minutes. Carefully tilt skillet toward you so that oil pools at front edge of pan. Spoon hot oil over egg whites, especially anywhere they are still translucent, while avoiding yolks, until whites are set, about 1 minute more. Transfer to a paper towel–lined plate to blot oil; season with salt and pepper.