Marinate chicken: add mayonnaise to a small bowl or plate. Mix salt, black pepper, garlic powder, and paprika into the mayonnaise and stir until everything is combined. With your hands, rub the mayonnaise-based marinade onto each piece of chicken. You can do this in a zip-top bag or right in the bowl where you made the marinade. Just make sure the chicken is fully coated. Marinate the chicken in the refrigerator for 1 to 2 hours.

Fried green tomatoes: preheat oven to 200 F (93 C). 

Get three bowls or pans to set up your dredging station. One bowl/pan will contain the all-purpose flour and Cajun seasoning. The second bowl will contain the buttermilk, egg, and hot sauce (if using). The third bowl will be the corn meal, breadcrumbs, salt, and black pepper. 

Over medium-high heat, place a medium pot or pan with 1 to 2 inches of vegetable oil. A thermometer will help because we're trying to get it to 350 degrees F (175 C). 

While the oil is heating up, slice your green tomato and lightly salt the slices. Bring them to your dredging stations. 

Each green tomato slice needs to be coated in the first bowl with the seasoned flour, then the second bowl with the buttermilk and egg mixture, and then finally coated fully with the cornmeal and breadcrumbs. During each station/bowl you need full coverage. No dry or wet spots. Make sure everything is fully coated in each bowl. 

Leave each tomato slice in the last bowl with the cornmeal and breadcrumbs until the oil is to temperature. 

When the oil is hot, place two or three coated green tomato slices in the oil. Do not crowd the pan with too many tomato slices, fry in batches if you need to. 

Fry for 2 to 3 minutes until the coated slices have the proper color of browning and then remove to a cooling rack in a sheet pan or a paper towel lined plate. 

After the tomato slices have cooled for about five minutes move them to a sheet pan and then into the preheated oven to stay warm while you grill the chicken.

Alabama white sauce: add mayonnaise, apple cider vinegar, ground black pepper, garlic powder, sugar, prepared horseradish, salt, and lemon juice to a medium sized bowl and whisk until everything is combined fully. This is your Alabama white sauce. Add to a sealed container and store in the refrigerator for around a week or so. 

Grill chicken: preheat grill for 10 minutes. 

Pull chicken out of the container with the marinade and cook on grill grates for 4 to 5 minutes per side for thighs and 5 to 6 minutes per side for chicken breasts. If you have very large chicken breasts you might want to grill for 7 minutes. If you have a probe thermometer you should be looking for 165 degrees F (73 C). You should be able to pull the chicken off the grill 5 degrees less than your target temperature and it will carry over cook those extra degrees. 

Once the chicken has cooked on both sides, remove it from the grill to a plate to rest. While the chicken is resting you can get everything ready to build your sandwich. 

Toast your buns if desired.

Sandwich assembly: add a tablespoon or two of Alabama white sauce to the top of each of your grilled chicken thighs or breasts. Using a spoon, make sure each piece is fairly coated with sauce.

Add a tablespoon of Alabama white sauce to the bottom of each bun. Top the sauce with chopped lettuce. 

Add your Alabama white sauce coated piece of chicken onto the bed of lettuce and top the chicken with a fried green tomato slice.

Cover everything up with the top bun and serve.