Season chicken breasts with salt and pepper. Add oil to a large skillet. Over medium-high heat, brown chicken on both sides; add ½ cup water, cover and cook until chicken is no longer pink in the middle. Remove chicken from pan and cool slightly. Shred chicken and return to skillet. Add jar of Southwest Slow Cooker Sauce and simmer 15 minutes. Break off lettuce leaves and fill with chicken mixture; top with cheese, tomatoes and a drizzle of ranch dressing.