Lightly pound the thickest part of the chicken breasts so that they are an even thickness of about ½ inch. While pounding, try to work the chicken breast into the shape of the bread.
Sprinkle each breast evenly on both sides with ½ teaspoon per side of the Poultry Magic, patting it in gently.
Place the oil in a nonstick skillet over high heat until the oil is just beginning to smoke, about 4 minutes. Put the chicken breasts in the skillet and cook until the underside is bronze in color, 2 to 3 minutes. Turn the breasts over and cook until done, 2 to 3 minutes more. Transfer the breasts to a plate while making the sandwiches.
Cut the bread in half lengthwise. Spread the mayonnaise (a little or a lot, as you prefer) on the inside of each slice. Arrange the chicken on the bottom half (if the chicken does not fit comfortably, it may be cut into several pieces). Top with lettuce leaves, tomato slices and caramelized onion, then top with the remaining slice of bread. Cut the finished sandwich in half crosswise to make 2 servings.