Cut chicken breasts into bite-size pieces and season with salt and pepper.
Heat olive oil in a 2-3 quart saucepan over medium heat.
Add chicken and cook until no longer pink. Remove from pan with a slotted spoon and set aside.
Add butter to the same pan and add onion, carrots, and celery. Cook for 5 minutes or until onions are translucent.
Add garlic, thyme, cayenne, salt, and pepper. Cook for another 3 minutes.
Add flour to the vegetables and cook for 1 minute longer.
Slowly add chicken stock to mixture, stirring constantly. Cook for 20 minutes or until vegetables are tender and soup thickens.
Add half-and-half and cook, stirring until thickened.
Return chicken to pan and add spinach, cooking for another 5-10 minutes.
Check seasonings and adjust salt, pepper, or cayenne to taste.
Serve hot with homemade croutons.