In a shallow bowl, whisk together the water, salt, and sugar until dissolved. Pound each chicken breast to a ½-inch thickness. Add to the brine and refrigerate for 30 minutes.
Meanwhile, prepare the Cajun mayo by mixing together the mayonnaise, ketchup, ¼ tsp blackened seasoning, and hot sauce in a small bowl; cover and refrigerate until ready to use.
Remove the chicken from the refrigerator and pat dry with paper towels. Brush each chicken breast with olive oil, then sprinkle both sides with blackened seasoning (about ½ tablespoon each).
Preheat grill to medium-high heat.
Grill chicken breasts until blackened and chicken is cooked through, about 3 to 5 minutes per side. Remove from grill and allow to rest for 5 minutes.
While the chicken is resting, place rolls on the grill, sliced side down, and grill until lightly toasted.
Remove rolls from grill and spread on the Cajun mayo. Top with the blackened chicken, tomatoes, and lettuce. Serve immediately.
Note: both the blackened chicken and the Cajun Mayo are fairly spicy, so you could also swap regular mayonnaise or ranch dressing for a more mild flavor.
Recipe adapted from: http://tonjastable.com