Put the chicken thighs, garlic, and rosemary in a large casserole dish. Drizzle with the olive oil and sprinkle with the salt.
Bake for 35-45 minutes or until the chicken is 160 degrees when checked with a meat thermometer.
Carefully pour the cooking liquid into a small saucepan. Add the salt, pepper, and rosemary. Sprinkle with the xanthan gum. Cook over medium heat, whisking, until it has thickened. Season with additional salt and pepper if desired. Serve with the rosemary chicken.