Season short ribs liberally on all sides with sea salt and cracked black pepper.
Heat coconut oil in a large, deep skillet over medium-high heat and brown short ribs and onions in batches on all sides, 5 to 8 minutes. Place ribs and onion in a slow cooker. Add celery, carrots, and cinnamon stick.
Stir beef broth, coconut aminos, fish sauce, ginger, garlic, salt, and pepper together in a bowl and pour over ingredients in the slow cooker. Cover with a lid.
Cook on Low until ribs are cooked through and flavors are well combined, 6 to 8 hours. Remove ribs. Raise heat to High and stir in arrowroot powder. Cook until sauce is slightly thickened, about 5 minutes. Serve ribs topped with sauce and chopped green onions.