Combine the spices for the spice blend, mix well, and set aside.
Gently pound the chicken breasts to an even thickness. Drizzle oil over each piece, coating evenly. Generously dust the spice blend over both sides of the chicken, rubbing it into the oil to form an even coating.
Begin heating a skillet over medium-high heat (use a cast iron if available), add a very generous swirl of oil to coat the pan. Once hot, cook the chicken until cooked through and blackened on the outside. I cook it for 6 to 8 minutes, flipping every 2 minutes or so.
Remove the chicken to a plate when done, and allow it to rest for a few minutes. You can use it whole on the sandwich, or chop it into chunks now.
Optional: Toast the buns. While the chicken cooks, heat a griddle or skillet over medium heat. Split the bun and butter the cut sides, then place on the griddle until golden, about 2-3 minutes. Set aside.
Assemble the sandwich: Spread spicy mayo on the bottom bun, top with lettuce, tomato, onion, the chicken, and cheese. If desired, you can place the chicken and cheese under a broiler to melt the cheese before assembling the sandwich.
Notes
Each chicken breast piece should be ⅓ to ½ pound in size. I typically shoot for about ⅓ lb. You can use small chicken breasts, or filet a larger breast into two pieces before using. You can use premade blackening seasoning instead of mixing your own. You'll need about 1.5 tablespoons for this recipe.