Heat oil in a large cast-iron skillet over medium high heat. Sprinkle ¾ teaspoon salt, Italian seasonings, crushed red pepper and black pepper all over the salmon. Lay salmon fillet in the hot oil. Let cook, undisturbed until browned and crispy along the bottom edge, and the skin is opaque about half way up the side of the fillet, 4 to 6 minutes, depending on thickness. Flip salmon over and remove skillet from heat. Allow salmon to remain in hot skillet to allow the carry-over heat to continue cooking the other side to desired doneness, about 4 minutes more.
Meanwhile, peel pit and mash avocado with basil, lime juice and the remaining ¾ teaspoon salt. Serve salmon topped with avocado mash sprinkled with scallions if desired.