Season fish liberally on one side with blackening spice and a small amount of salt and pepper. Set aside.
Place cast iron skillet on a burner and turn on high for about 3 minutes, until it becomes very hot. Turn off flame, add oil and place fish, seasoning side down, in pan.
Turn flame back on and cook fish, seasoning side down, until translucent, about 5 minutes. The fish will turn white except for a small amount of pink in center. At this point, turn off heat and flip fish over in pan.
Allow to sit in pan for about 1 minute. Remove from pan and place on a paper towel to absorb some of residual oil.
Heat three tortillas in microwave for 30 seconds, to warm. Spread a small amount of remoulade on each tortilla, then lay three roughly equal-sized portions of fish on individual tacos. Top with lettuce (or cabbage) and tomato, then dollop a generous portion of remoulade on top of tacos. Makes 4 servings of 3 tacos.