In the same pan, add the sliced yellow onion, mushrooms and seasonings (salt, pepper, onion powder, cajun seasoning, thyme). Still to combine and cook until onions are mushrooms are tender, about 4-5 minutes.
Meanwhile, in a separate bowl, add the coconut aminos, yellow mustard, and apple cider vinegar and whisk to combine until no chunks. Set aside.
Next, add the arrowroot powder to the onion and mushroom mixture and stir to combine.
Slowly add beef broth, stirring continuously to combine. Now add in the coconut aminos, mustard, ACV mixture, stir to combine, and let simmer until sauce begins to thicken, about 3-5minutes.
Add back in the meatballs and nestle each in the sauce.
Transfer to oven and let cook until meatballs are cooked through to an internal temperature of 165°, about 10 minutes.
Remove from oven and garnish with chopped fresh parsley.