Clean and spray grates of a gas grill with non-stick cooking spray and set heat to high. Close lid until grill comes to temperature.
Meanwhile, prepare the gremolata by combining the parsley, lemon zest, preserved lemon rind, and garlic a small bowl. Season with salt and black pepper, to taste, and set aside.
Brush tuna steaks with olive oil and sprinkle with Mediterranean-spiced sea salt and black pepper, to taste.
Open grill and 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. Serve with our grilled marinated asparagus or other side of your choice. Enjoy!