Add the celery and carrots to the bottom of a 6-quart slow cooker. This creates a simple rack for the turkey breast to sit on.
Pour in the chicken broth and add the bay leaf.
Pat the turkey breast dry with paper towels. Leave the netting on if there is one, then place the turkey breast on top of the vegetables.
Rub the softened butter evenly over the turkey breast.
In a small bowl, stir together the salt, pepper, parsley, and thyme. Sprinkle the seasoning evenly over the turkey.
Cover and cook on low for 3 to 4 hours, until the internal temperature of the turkey breast reaches at least 165 degrees F.
Using large forks, carefully transfer the turkey breast to a cutting board or a 9×13-inch pan to catch any juices. Tent loosely with foil and let rest while preparing the gravy.
If making the gravy, strain the cooking liquid through a fine-mesh strainer into a bowl to collect the turkey drippings.
Pour 1 cup of the turkey drippings back into the slow cooker.
In a small bowl, melt the butter. Stir in the gluten free flour to form a thick roux.
Whisk the roux into the turkey drippings until smooth. If the gravy is too thick, add a small splash of turkey drippings or chicken stock until it reaches your desired consistency.
Notes
Note 1: Use a plain, unseasoned boneless turkey breast that is fully defrosted. Note 2: Make sure the turkey drippings do not have too much fat, which is why a fat separator works great. Otherwise, the gravy will not thicken. Low FODMAP Notes: Low FODMAP ingredient information is based on the Monash University Food App and FODMAP Friendly App testing results.