Rinse the parsley and remove as much water as possible by shaking into the sink (hold onto the stems). Pull the leaves from the stems and place them in the food chopper or processor. Cut the lemon in half and add the juice and zest from one half into the food chopper. Save the other half of the lemon for garnish. Also add the garlic, olive oil, salt and pepper. Turn the chopper on until the parsley and garlic are finely minced and the mixture is well combined.
Place the fish fillets (thawed) to a bowl or shallow dish and pour the lemon-garlic-parsley mix over top. Make sure both sides are covered. Some may run off but that is okay. Spray the grill surface with non-stick spray and preheat for 5 minutes.
Place the fish fillets in the hot grill, close the lid and set the timer for 2 minutes. Check the fish after two minutes. If you want the fish more browned, cook for one additional minute. Carefully remove the fish from the grill (it will be delicate). Serve with additional slices of lemon if desired.