Set the Instant Pot to Saute mode and add the olive oil. Working in two batches, sear the flank steak slices for 1-2 minutes per side until lightly browned. Remove and set aside.
Add the minced garlic and grated ginger to the pot. Saute for 30 seconds until fragrant.
Pour in the soy sauce, water, and sesame oil. Add the brown sugar and red pepper flakes. Stir well to combine and scrape up any bits from the bottom.
Return all the beef to the pot and stir to coat in the sauce. Lock the lid, set the valve to Sealing, and cook on High Pressure for 12 minutes.
Perform a quick release by carefully turning the valve to Venting, keeping hands and face away from the steam. Remove the lid once the pin drops.
Switch to Saute mode. Stir in the cornstarch slurry and let the sauce simmer for 2-3 minutes, stirring gently, until it thickens and coats the beef.
Garnish with sliced green onions and sesame seeds. Serve over steamed white rice.