1Flatten each chicken breast half to 1/4-inch thickness between sheets of plastic wrap or waxed paper. Sprinkle with salt and oregano.
2Set oven control to broil. Place chicken on rack in broiler pan. Broil with tops 4 to 6 inches from heat 15 to 20 minutes, turning once, until no longer pink in center.
3Cut bread horizontally in half; cut into 6 wedges. Spread pesto on cut sides of bread. Layer chicken, tomato and spinach on bottom wedges. Top with top wedges.