Season the chicken breast with salt and pepper.
Heat olive oil in a skillet over medium heat.
Cook the chicken breast for 6-7 minutes on each side until fully cooked.
Remove the chicken from the skillet and let it rest.
Toast the sandwich bun lightly in the same skillet.
Spread mayonnaise on the bottom half of the bun.
Slice the cooked chicken breast and place it on the bun.
Layer with lettuce, tomato, cucumber, and red onion.
Top with the other half of the bun and serve immediately.