Add the blackening seasoning and oil to a large bowl or plate and mix until combined.
Next, dredge the cod fillets on all sides in the mixture until they are completely coated. Set aside.
Heat a large frying pan over medium heat and add the fish and cook until browned and cooked throughout, about 3 to 4 minutes per side. Feel free to add more oil if it seems that the fish is sticking.
To Plate: Place the cooked blackened fish on a toasted hoagie bun and serve with lettuce, tomatoes, onions and tartar sauce.