Add onion, celery, carrots, garlic, thyme, bay leaf, broth, wild rice, salt, and pepper to your slow cooker.
Cover and cook on high for 3 to 4 hours or low for 7 to 8 hours.
When done, remove bay leaf and thyme stems.
In a saucepan over medium heat, add the butter. Once melted, stir in the flour and cook for 1 minute. Slowly whisk in the milk until no lumps remain. Continue to cook another minute, or until thick and creamy.
Add the flour mixture to the slow cooker. Stir to combine. Season to taste with additional salt and pepper.
Notes
The flour mixture might be really thick. This is okay. Once you add it to the soup, it’ll liquify and make the soup nice and creamy. Just use a fork to help it mix into the soup.