Lightly grease the slow cooker with olive oil.

Add the diced onion, carrots, and celery to the slow cooker.

Place the chicken breasts on top of the vegetables.

Add lentils, garlic, thyme, rosemary, and the bay leaf.

Pour in the chicken broth and add the diced tomatoes.

Cover and cook on low for 6 hours or high for 3 hours, until the chicken is fully cooked and lentils are tender.

Remove the chicken, shred it, and return it to the soup.

Stir in the fresh spinach until wilted.

Remove the bay leaf and mix in the Greek yogurt until smooth and creamy.

Season with salt and black pepper before serving.