Preheat a grill to medium-low heat, about 350 degrees F.

Season the tenderloin on all sides with the Essence, salt and pepper. Brush the pork with the oil, transfer to the grill, and cook until the thickest part of the tenderloin registers 140°F on an instant read thermometer, 18 to 20 minutes. Transfer the pork to a small cutting board, let it rest for at least 10 minutes, and then slice it crosswise into ¼-inch-thick slices.

Place the mayonnaise in a small bowl. Add the Creole mustard, green onion, garlic, and cayenne pepper and stir to combine.

Using a serrated knife, halve the loaf of bread lengthwise. Remove and discard some of the dense bready portion from one side, leaving about 1/2 inch of thickness. Leave the other half intact. 

Generously spread the mayonnaise spread along both halves of the loaf. On the bottom half, layer the ham, mortadella, and pork tenderloin. Spread an even layer of sliced onion on top of the pork; then layer the pickle slices and Swiss cheese. Top with the other half of the bread, and press to close. Cut the loaf into 4 sandwiches.

Brush the outsides of the bread with the softened butter and wrap each sandwich in aluminum foil, sealing the edges well.

Heat a grill over medium heat, and place the wrapped sandwiches on the grill. Place a weighted sandwich press or a small skillet on top of each sandwich and cook until the sandwiches are warmed through, cheese is melted, and the outsides are crispy, turning the sandwiches mid-way through cooking, 8 to 10 minutes. Once unwrapped, if the sandwiches are not crispy enough for your liking, they may be returned to the grill briefly (unwrapped) to crisp up before serving. Take care, as the buttered bread will burn quickly when in direct contact with the grill grates.

Serve the sandwiches warm.