In a large measuring cup, whisk together the soy sauce, beef broth, water, fish sauce, garlic, green onions, sugar substitute, and ginger until evenly combined.
Place the sliced flank steak in a resealable bag or shallow dish. Pour the marinade over the steak, turning to coat all pieces. Seal the bag or cover the dish and refrigerate for at least 30 minutes, or up to 4 hours for the deepest flavor.
Lightly spray or oil the Instant Pot insert. Add the marinated steak and all marinade liquid to the pot.
Secure the lid, set valve to 'sealing,' then select 'high pressure' and set timer for 10 minutes. While the beef cooks, steam the broccoli until crisp-tender, then set aside.
When the cooking time ends, perform a quick release of the steam. Transfer beef to a plate, leaving all cooking liquid in the Instant Pot, and cover loosely to keep warm.
Press 'cancel,' then select 'sauté.' In a small bowl, whisk together the arrowroot powder and water until smooth. When the sauce starts to boil, slowly whisk in the slurry, stirring continuously as the sauce thickens. This typically takes about 1–2 minutes.
Add reserved steak and steamed broccoli to the thickened sauce in the pot. Toss gently to coat everything in sauce and let it heat through for 2–3 minutes.
Serve immediately.