Cut scallion or spring onion stalks roughly in half crosswise to separate white and light green bottoms from dark green tops. (If using spring onions, slice the bottom pieces in half lengthwise to make thinner.) Cut scallion bottom pieces in half crosswise to make shorter segments. Set separated dark green tops aside.


Preheat a gas or charcoal grill to medium-high (400°F to 450°F). Arrange 4 (12-inch) square sheets of aluminum foil on a work surface.

Arrange 8 scallion bottom segments in a single layer on the center of each foil square. Sprinkle grouper fillets evenly with salt, pepper, and paprika, and place 1 fillet on top of scallions on each foil square. Top each fillet evenly with lemon slices to cover completely. Place 2 butter cubes over each layer of lemon slices.


Bring together top and bottom of each foil square above the grouper mixture, and fold foil tightly over itself 3 times. Fold each outer side of foil packet in toward center 3 times, pinching to seal the packet. The packet should be airtight but with space above the food inside the packet.


Place foil packets on unoiled grill grates; grill, covered, until a thermometer inserted into grouper registers 135°F, 8 to 10 minutes.

Transfer foil packets to a baking sheet, and let rest 5 minutes (the grouper will continue to cook until the internal temperature reaches 140°F to 145°F). Meanwhile, thinly slice 1/4 cup of the reserved dark green scallion tops; toss with parsley in a small bowl until combined.


Carefully unwrap each foil packet, reserving liquid in bottom of each packet; discard lemon slices. Arrange grilled scallion segments and grouper evenly on 4 plates, and pour reserved liquid over each fillet. Top fillets with sliced scallion-parsley mixture. Serve immediately alongside baguette or rice.