The key to this simple recipe is using incredibly fresh fish so that wine, salt, and lemon enhance its flavour. While you could just cook the fish directly in the skillet or pot, sealing it in parchment paper steams the fish more gently and evenly, making it harder to overcook.
Per serving:
On 6 large pieces of parchment paper, divide asparagus spears and green onions evenly.
Season fish with salt and pepper and place on top of vegetables.
Curve up the rims of parchment a little so the liquid doesn’t run off, then sprinkle oil and wine or water overtop. Top with lemon slices.
Wrap each packet with the seam on top, rolling seam over itself once or twice to seal tightly, leaving a little room inside parchment for steam to circulate.
Place packets in cast iron skillet or fire-safe pot on grill rack over a medium flame for 10 to 15 minutes, rotating packages occasionally for even cooking.
Cooking time will depend on how hot your fire is and how close the fish is to the flames. Test one fillet with a fork after 10 minutes; if not just barely flaking, reseal parchment and continue cooking for a few minutes longer.