Heat a grill to medium-high heat.

Place the chicken breasts on a baking sheet or large dish and pat dry. Drizzle lightly with olive oil and rub the chicken to coat evenly.

Sprinkle half of the spice mixture over the tops of the chicken breasts, turn the chicken over, and sprinkle the remaining half of the spice mixture on the bottom side of each breast so the chicken is evenly coated in Cajun seasoning.

Place the chicken breasts directly over the heated side of the grill and grill for about 5-6 minutes per side, depending on the thickness of your chicken, until the internal temperature of the breasts reach 160 degrees using an instant read meat thermometer. You can also check whether the chicken is done by making a small cut into the thickest part to see if it is opaque in the center, or pressing on the thickest end of the chicken with your finger to see if there is any resistance. If there is no resistance, the chicken is done. Remove from the grill and rest for 5 minutes.

Heat a large pan or griddle over medium heat. Separate the top and bottom buns and spread the cut sides of each bun with a little mayonnaise. Lay each bun mayonnaise side down in the pan or on the griddle, working in batches as necessary, and cook until toasty, then remove to a plate.

Prepare each sandwich with a toasted bun, 1 grilled Cajun chicken breast, and slices of lettuce, tomato, and onion, along with additional mayo or aioli. You could also add cheese if you would like. Serve immediately!