To prepare the stuffing:

First sauté diced onion in 1/8 stick of butter. When then onions are translucent add a touch of oregano, Cajun seasoning, garlic clove, a touch of basil, and a dash of black pepper. Add the shrimp (you can also use popcorn or salad shrimp) and crawfish tail meat. Sauté these items until the seafood is just cooked. Then add breadcrumbs according to taste. You can use dry bread in this mixture. Add 1/2 stick of melted butter. Allow to cool for 20 minutes.

To prep tenderloin:

Lay tenderloin on its side and determine the direction of the "grain" of the meat. Then stand the tenderloin on end, cut a "cross" using a small steak knife in the center-top of the tenderloin around of 1/16 of an inch from the edges. Be careful not to cut too deep and open the bottom. Stuff tenderloin with 1/4 cup of stuffing mixture and roll it into a roll about 4'' long. Load it into the tenderloin and lightly flatten excess on top. Lightly roll tenderloin in Cajun seasoning and you are ready to cook it.

Options to cooking:

The ideal way is to grill at a medium high heat on all sides saving the top for last. It takes about 20 minutes to cook properly. Another way is to place on a cast iron skillet heated to 700-750 degrees for about 4 minutes a side. Then finish cooking in a grill or your oven. Be aware that if you use a skillet you must do it outside or in a ventilated kitchen with a hood that exhausts to the outside. Serve medium rare to medium.