Slice flank steak thinly against the grain.
Toss the beef with cornstarch until evenly coated.
Turn Instant Pot to sauté mode and heat oil.
Add garlic and ginger, cook until fragrant.
Pour in soy sauce, water, brown sugar, and black pepper. Stir well.
Add beef and mix to coat in sauce.
Close lid and cook on high pressure for 20 minutes.
Let pressure release naturally for 20 minutes, then release remaining pressure.
Turn on sauté mode and simmer until sauce thickens.
Garnish with green onions and serve warm.