Prepare your campfire or camp stove. For a fire, let it burn down to hot coals with no flames. For a stove, set it to medium heat.
Lay out four large sheets of heavy-duty foil, each about 12 inches long. Lightly oil the center of each sheet.
Place one fish fillet skin-side down in the center of each foil sheet. Season with salt and pepper. Top with a pat of butter or a drizzle of oil, two lemon slices, and a sprig of dill.
Fold the foil over the fish to create a packet. Seal the edges by folding them over twice, leaving a little room for steam to circulate.
Place the foil packets directly on the coals or on the stove grate. Cook for 8-12 minutes, then carefully flip the packet with tongs and cook another 8-12 minutes. The fish is done when it flakes easily with a fork.
Remove from heat. Let rest for 2 minutes. Open the packet carefully—steam will escape. Serve directly from the foil for easy cleanup.