To butterfly the chicken: Slice the chicken breast down the middle horizontally lengthways to into 2 thin breast pieces. Season the chicken on both sides with salt and pepper. Coat with breadcrumbs on both sides pressing down firmly to coat well.
Heat half the oil in a nonstick pan over medium heat. Cook chicken until golden brown on both sides to an internal temperature of 160 degrees is reached using a thermometer. Set aside on a paper towel lined dish to drain excess oil.
Whisk together the remaining oil and vinegar in a bowl and toss with spring mix, onion, and tomatoes. Season with salt and pepper to taste.
Place the spring mix over top of the chicken and serve. Garnish with cheese and serve with bread.