Common thickening agents for soups and stews traditionally have included flour, cornstarch, arrowroot, and xanthum gum. None of these options are 100% Bright Line Eating friendly.
You may consider using psyllium husk, chia seeds, or ground flax seeds as thickeners. In addition, many Bright Liners will puree a portion of beans and vegetables to add more thickness to their soups or stews. Remember to count anything you add for thickening as part of your food plan serving portion, if applicable. For example, psyllium husk does not need to be counted. However, chia seeds or ground flax seeds would need to be counted as part of your protein or fat serving.