Heat butter in a pan over low heat and add celery onions and carrots. Cook until just beginning to soften.
Add garlic and thyme, cook until fragrant; about 30 seconds.
Add chicken stock, bay leaf, salt and pepper, chicken breasts and wild rice. Bring to a boil, lower heat and simmer for about 45 minutes until chicken is cooked and rice is tender.
Take out chicken and shred. Once shredded, add back into pot along with the heavy cream.
Make the roux by mixing 3 tablespoons of butter and 3 tablespoons of flour. Stir into soup and add the spinach. Bring to a gentle boil. Simmer for a few minutes until soup begins to thicken. Salt and pepper to taste.
Notes
If using frozen spinach, I suggest adding it to the pot when you add the chicken and heavy cream.