Heat a 12-inch cast iron frying pan over medium heat.
Fry the pancetta, turning it frequently so that it browns and crisps on all sides; this takes about 10 minutes.
Remove it with a slotted spoon and drain it on paper towels.
Leave the heat on and the renderings in the pan.
Add the potatoes and don’t move them for a couple of minutes. Use this time to season them well with salt and pepper.
Once they’ve gotten a little brown underneath, begin flipping and turning them, then letting them cook again for a few minutes.
When the potatoes are about three-fourths as crisped and brown as you’d like them — this takes about 15 minutes — add the onion.
Cook for an additional 5 minutes.
Add the asparagus, cover the pan and cook for 5 to 8 minutes, or until crisply cooked.
Remove the lid, return the pancetta to the pan for another minute, to reheat.
Taste for seasoning and adjust if needed. Serve immediately.
