In a large pot over medium-high heat, combine the chicken breast, red lentils, chicken broth, onion, carrots, celery, garlic, thyme, rosemary, and the bay leaf.
Bring the liquid to a boil, then immediately reduce the heat to low and cover the pot with a tight-fitting lid.
Simmer the mixture for 20 to 25 minutes, or until the lentils have completely softened and the chicken is cooked through to an internal temperature of 165 degrees Fahrenheit.
Remove the pot from the heat and carefully fish out the bay leaf and discard it.
Using a high-speed blender or an immersion blender directly in the pot, process the soup until it reaches a completely smooth, creamy, and uniform consistency.
Stir in the fresh lemon juice, sea salt, and black pepper, adjusting the seasoning if necessary.
Pour the warm soup into a bowl and serve immediately while hot.