Cut focaccia in half horizontally. Set bread aside.
Place each chicken breast half between two pieces of plastic wrap. Using the flat side of a meat mallet and working from the center to the edges, lightly pound chicken to 1/4-inch thickness. Remove plastic wrap. Season chicken with salt, garlic powder, and black pepper.
In a large skillet cook chicken in hot oil over medium heat for 6 to 8 minutes or until no pink remains, turning halfway through cooking. Remove from skillet. Spread each chicken breast half with 1 tablespoon of the pesto.
To assemble sandwich, layer lettuce on the bottom of the focaccia. Top with cheese slices, chicken, roasted peppers, and basil leaves. Top with focaccia top. Cut into 6 wedges.