Wipe out skillet, return it to stovetop, and lower heat to medium. Add 1 tablespoon plus 2 teaspoons (25ml) pork fat (or vegetable oil) and pulled roast pork shoulder to now-empty skillet, and arrange pork in a single layer. Season lightly with salt and pepper, and cook, stirring occasionally, until pork is golden brown and crisp on all sides, 8 to 10 minutes. Using a slotted spoon, transfer pork to a plate and set aside.
Add remaining 2 tablespoons (30ml) pork fat (or vegetable oil) to skillet and increase heat to medium-high. Add potatoes and arrange in as even a single layer as possible. Season lightly with salt, and cook, stirring and tossing occasionally, until potatoes are deep brown and crisp on all sides, about 20 minutes. Add coriander, chili powder, and cumin and cook, stirring frequently, until fragrant, about 30 seconds. Return pork, asparagus, and serrano to the skillet, along with any drippings reserved from roast pork shoulder (if you have saved them), toss to combine, and continue to cook until all components are heated through, about 2 minutes. Season with salt and pepper.
