For the lionfish: Preheat a grill or grill pan. Place 2 lionfish fillets each into the center of two 10-by-10-inch aluminum foil squares. Combine the garlic salt, oregano, paprika, cayenne and black pepper in a small bowl, and then coat both sides of the fillets with the spice mix. Top the fillets with the butter cubes, and then tightly wrap the foil squares to form 2 pockets. Place the pockets on the grill and cook until the fish is tender and flakey, about 6 minutes.
For the potato salad: Combine the celery, eggs, potatoes and onions in a large mixing bowl. Mix together the mayonnaise, relish, mustard, vinegar, dill weed, black pepper, lemon juice and salt in a separate bowl, and then pour over the potatoes and toss to combine. Taste and adjust the seasoning, if necessary.
To serve, transfer the fish to plates, drizzling the melted butter on top. Scoop some potato salad alongside and enjoy.
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