Heat grill or skillet to medium-high.
Cut chicken breasts in half crosswise to make 4 pieces. Pound each piece between sheets of plastic wrap with a heavy skillet to an even 1/4-inch thickness, being careful not to tear meat.
Spread pesto on both sides of chicken. Brush and oil grill grate or skillet then grill chicken until no longer pink, about 2 minutes per side.
Cut bread in half through side to make two long slabs. Cut crosswise into 4 squares. Spread goat cheese or additional pesto on bottom side of each sandwich. Divide arugula, scallions and roasted bell peppers among sandwiches. Top with grilled chicken. Put on top of sandwich and cut in half diagonally to make 2 triangles per sandwich. Serve warm.
Tip: Refrigerated pesto tends to taste better than jarred varieties. Look for it in the refrigerated pasta case of your grocery store. When pounding the chicken, sprinkle it with a few drops of water to prevent it from sticking to the plastic wrap. Begin pounding from the thickest part of the meat outward to encourage the meat to spread as it thins.