Lay fish fillets side by side, touching, on a piece of aluminum foil that is 4 in. longer than the fillets on both ends, and 6 in. wider on both sides. In a bowl, combine celery seed, garlic, pepper, salt and red onion. Spread evenly over fish. Arrange dill sprigs over the onion mixture. Top with lime slices. Evenly distribute butter between fillets and on top of limes. Draw up the foil around the cod from the sides and start rolling it together down towards the fish, creating a tent/pouch. Draw up both ends of the foil and fold over the top of the fish to seal the ends. Place the fish package on the grill for about 10 minutes, or until cod is cooked through (this will depend on the thickness of your fish). It should flake easily with a fork when done. (Be very careful opening the foil package as a lot of steam will escape.) Makes 4 servings.