Preheat a skillet to medium and add 1 tablespoon olive oil. Once hot, sauté the onions and garlic until soft, 3-5 minutes.
Pour the onions and garlic into the crock of a large 6-quart crockpot. Add the carrots, peppers, whole chicken breasts, dried lentils, rosemary, chicken broth, 1 1/2 teaspoons salt, and 1/2 teaspoon cracked pepper. Stir and cover. Then set the slow cooker on low for 10-12 hours (or high for at least 6 hours).
Anytime during the slow cooking process, place the chimichurri ingredients in a food processor. Puree the ingredients until mostly smooth. Then salt and pepper to taste. Set aside until ready to serve.
Once the lentils are soft and the broth has thickened. Remove the chicken breasts with tongs and shred the meat with two forks. Stir the shredded chicken back into the soup. Taste and add salt as needed. Then ladle the soup into bowls and allow each person to stir in as much chimichurri as desired.
NOTES: All slow cookers vary slightly in temperature. Know thy crockpot and adjust the cooking time accordingly.