Spray grates of grill with non-stick cooking spray and set heat to high.
While grill is heating, prepare the gremolata by combining the parsley, lemon zest, garlic and preserved lemon (if using) in a small bowl. Season with salt and black pepper, to taste, and set aside.
Brush tuna steaks with olive oil and sprinkle with Italian seasoning, sea salt and black pepper, to taste.
Place tuna steaks and lemon halves over direct heat. Cook tuna for 2 minutes per side for rare. Increase time slightly for medium, but do not overcook.
Remove tuna and grilled lemon halves from heat and serve immediately topped with some gremolata.