Slice French bread in half lengthwise and hollow out each slice, leaving a scant 1-inch shell. Brush inside of both slices with the olive oil, then sprinkle evenly with red wine vinegar. Line the bottom half of bread with the basil leaves, then spread with olive salad and sliced onions. Season with black pepper, then top with protein of choice. Layer on cheese slices, then sliced tomato, and top with remaining bread. Top with a plate or baking sheet and press down firmly to compress, then add a heavy object such as a cast-iron skillet to weight it down. Set aside for for 20 minutes to marinate.

While the sandwich marinates, preheat oven to 375ºF with 2 cast-iron baking sheets or skillets on a lower rack.

After 20 minutes, carefully transfer marinated sandwich to one of the preheated pieces of cast-iron, carefully top with the other pans, and bake until toasty, 10 to 15 minutes. Slice crosswise into triangles and serve hot.