Slice the onion in very thin slices and add to the slow cooker.
Mince the garlic and add it into the slow cooker.
Add in the red pepper flakes, parsley flakes, lentils, chicken broth and chicken.
Cover the slow cooker and cook on LOW for 5-6 hours or on HIGH for 2 1/2-3 hours.
Remove the lid and remove the chicken and place on a cutting board. Shred the chicken and return to the slow cooker.
In a small bowl whisk together the egg yolks and lemon juice. Stir into the slow cooker.
Salt and pepper to taste
Ladle soup into bowls and serve.