Place each chicken breast between two sheets of plastic wrap. Using a meat mallet or rolling pin, gently pound to an even 1/2-inch thickness. This ensures quick, even cooking.

Season both sides of the chicken with salt and pepper. Let rest at room temperature for 10 minutes—this helps the seasoning adhere and the meat cook more evenly.

Heat a large skillet over medium-high heat for 2 minutes. Add the olive oil and swirl to coat. The oil should shimmer but not smoke.

Carefully place the chicken in the skillet in a single layer. Do not crowd—cook in batches if needed. Let sear undisturbed for 4-5 minutes until a deep golden crust forms. The chicken should release easily from the pan; if it sticks, it needs another minute.

Flip the chicken using tongs. Cook for another 3-4 minutes, until the internal temperature reaches 160°F (71°C). Remove chicken to a plate and tent loosely with foil. The carryover heat will bring it to 165°F.

Reduce the heat to low. Place the split buns cut-side down in the same skillet for about 1 minute, until lightly toasted and golden. Remove and set aside.

Spread about 1 tablespoon of pesto on the cut side of each bun bottom. Place a chicken breast on each. Top with cheese (if using), tomato slices, and a handful of arugula. Drizzle with balsamic glaze if desired. Cap with the bun top and serve immediately.