Heat the butter and cook the chopped celery, carrots and onion for 5-7 minutes over low heat, then add garlic and thyme and cook for 30 seconds longer.
Add the chicken stock, one bay leaf, a pinch of salt and pepper, chicken breasts, wild rice and bring to a boil. Then lower the heat and simmer for 45 minutes until the rice is tender. Take the chicken breasts out and shred with two forks, then add it back to the soup together with the double/heavy cream.
Make the roux by mixing melted butter with the flour, then stir it into the soup, bring to a gentle boil and let the soup simmer for a few minutes until it thickens. Stir in spinach if using. Add salt and pepper to taste and take it off the heat.