Start by trimming any excess fat from the chicken breasts. Cut each breast in half lengthwise to create thinner pieces that will cook evenly. Season both sides generously with salt and fresh ground black pepper.
Place the seasoned chicken breasts in the slow cooker. Pour the 2 cups of salsa over the chicken, ensuring each piece is well-coated.
Drizzle the fresh-squeezed lime juice over the salsa-covered chicken. This will add a beautiful tang to the dish as it cooks.
Cover the slow cooker and set it to low. Let the chicken cook for 6 to 8 hours, or until it reaches an internal temperature of 165°F (75°C). The longer it cooks, the more tender it will become.
Once the chicken is cooked, carefully remove it from the slow cooker and set it aside. Transfer the remaining salsa to a saucepan and simmer over medium heat until it reduces down to about 1 cup. This will concentrate the flavors.
Return the chicken to the slow cooker, pour the reduced salsa over it, and sprinkle the grated mozzarella on top. Cover and cook for an additional 15-20 minutes on low, just until the cheese is melted and bubbly.
