In a large, heavy-bottomed pot with a lid, heat the oil over medium-high heat. When the oil is shimmering, add the onion and saute until nicely browned, 5 to 6 minutes. Add the garlic, ginger, and Chinese five-spice and saute for another 30 seconds, until fragrant.
Add the broth, rice wine, and coconut aminos and bring to a boil. Season the beef with the salt and pepper and add it to the pot. Reduce the heat to low so that the liquid is just barely simmering. Cover and cook for 1 1/2 to 2 hours, until the beef is fork-tender.
In a small bowl, whisk together the tapioca starch with 1 cup of the stew liquid to create a slurry. Pour the slurry into the stew and stir.
Add the sweet potatoes and simmer for another 20 minutes, or until the potatoes are tender.
Add the bok choy and fish sauce and cook for another 5 minutes, until the bok choy is tender.
Serve the stew garnished with sliced green onions.