Sprinkle both sides of fish with the Cajun seasoning, and set aside.
While the seasoning marinates the fish, place avocado through lime juice in a blender of food process and blend until smooth. Season with salt and pepper, to taste, and add more lime juice (or water) if you want to thin it out some, or more Greek yogurt if you want it thicker. Refrigerate until ready to use.
In a small bowl, combine the chopped tomatoes, cilantro, jalapeno and lime juice. Season with salt and pepper, and set aside.
Heat olive oil in a large skillet over medium-high heat. Add the fish, and cook for 2-3 minutes per side, until nicely seared and flakes easily with a fork. You can either leave the pieces semi-whole, or break up into smaller pieces, whichever you prefer.
To assemble, warm tortillas, then top with lettuce, fish, salsa and avocado cilantro sauce. You can add additional cilantro on top as well, if you like!