Add the drained albacore tuna into a medium size bowl then add in the mayonnaise, Dijon mustard, sweet relish, red onion, Cajun seasoning, and black pepper.
Mix all of the ingredients together until they are fully combined. Set the tuna salad aside and toast the bread.
Add some butter or olive oil to a cast iron skillet and toast the bread on both sides until it is golden brown. Alternatively, you can use a toaster.
Once the bread is toasted, add equal portions of sliced avocado to each slice of bread. Then top the avocado with about ¼ cup of the prepared tuna salad and spread it out evenly.
Top the tuna with the lettuce. Cut the tunacado sandwich in half, and enjoy!