Season the chicken breasts with salt and pepper. Heat the olive oil in a skillet over medium heat. Cook the chicken for 6-7 minutes on each side, or until golden brown and cooked through. Let the chicken rest for a few minutes before slicing it thinly.
While the chicken is cooking, toast the bread slices in a toaster or on a griddle until golden and crispy. If you prefer, you can spread a little butter on the bread before toasting for extra flavor.
Spread a generous layer of pesto on one slice of toasted bread. If desired, spread a thin layer of mayonnaise on the other slice. Layer the sliced chicken, tomato slices, and arugula (or spinach) on top of the pesto.
Top with the second slice of toasted bread, cut the sandwich in half, and serve immediately. Enjoy with a side of chips, a pickle, or a simple salad for a complete meal!