For tuna herb mixture, mix macadamia nut oil, lime juice, marjoram, cilantro, mustard powder, chili powder, garlic powder, paprika, and salt in a small bowl.
Brush each tuna steak with herb mixture, patting into fish with fingertips. Cover and refrigerate for 1 hour.
For mango salsa, mix macadamia nut oil, lime juice, and cilantro in a medium bowl.
Stir in mango, bell peppers, shallots, chives, and jalapeño, if using. Season with salt and pepper. Set aside.
Heat a medium skillet over medium heat. You should not need extra oil for cooking because the fish was marinated in oil. Place tuna steaks in pan and sear, 1 to 2 minutes per side, depending on how cooked you prefer them. Don’t overcook; the tuna should be pink on the inside. Transfer tuna to a cutting board and slice against the grain.
Arrange the endive leaves on a platter and top with mango salsa. Place sliced tuna on the side of the platter.