Place the chicken breasts on a cutting board and pound them to an even thickness. This will help the chicken cook quickly and evenly.

Rub both sides of the chicken breasts with the olive oil.

In a small bowl, combine the garlic powder, onion powder, paprika, salt, and black pepper. Season both sides of the chicken evenly with the seasoning mixture.

Heat an outdoor grill or grill pan over medium-high heat.

Place the seasoned chicken breasts on the hot grill. Cook for 5 to 7 minutes on the first side.

Flip the chicken and cook for another 5 to 7 minutes, or until the chicken is cooked through and the thickest part reaches an internal temperature of 165°F.

If using cheddar cheese, place one slice on each chicken breast during the final minute of cooking. Allow the cheese to melt, then remove the chicken from the grill.

Place the hamburger buns cut-side down on the grill. Toast for 1 to 2 minutes, or until lightly golden. Remove from the grill.

Add the mayonnaise and Dijon mustard to a small bowl. Stir until smooth and evenly combined.

Spread the Dijon mayonnaise sauce over the toasted buns. Layer each sandwich with lettuce, grilled chicken, sliced tomato, red onion, cheddar cheese, and pickles, as desired.

Place the top bun over each sandwich and serve immediately.