In a bowl, combine mayonnaise, grain mustard, and lemon juice. Season to taste with salt. Set aside
In a bowl, combine crabmeat, lemon juice, sliced green chili, celery, and sliced onion. Season to taste with sea salt and freshly ground black pepper. Set aside.
Spoon the mayonnaise mixture evenly over the fillets of Snook. Set aside.
Lay 2 sheets of aluminum foil on a flat work surface, one on top of the other. Place a quarter of the onion mixture in the centre of the foil.
Top with the Snook fillet. Top the fish with a quarter of the onion mixture. Roll up the foil into a pouch and press the edges to seal firmly.
Preheat grill to medium high. (400°F to 450°F)
Place Snook pouches in the centre of the grill and close grill lid. Bake for 15 to 18 minutes turning once until fully cooked.
Carefully remove pouches from grill and allow to rest for 3 minutes.
Carefully open pouches allowing the steam to escape (be careful as this can be very hot) and serve immediately.