Cut the flank steak across the grain into roughly 1-inch pieces. Pat dry and season with the black pepper.
Select Sauté. Once the pot is hot, add the vegetable oil and sear the beef on two sides. Work in batches if needed, then return all the beef to the pot.
Add the sesame oil, ginger, and garlic. Cook, stirring, for a couple of minutes, watching closely to prevent scorching. Press Cancel.
Add the onion, mushrooms, broth, soy sauce, and vinegar. Scrape the bottom until all stuck-on browned bits are loosened. Do not add the zucchini or slurry yet.
Secure the lid and set the valve to Sealing as required by your model. Cook on High Pressure for 4 minutes. Allow additional time for the pot to reach pressure.
When the cycle ends, quick-release the pressure according to the manufacturer's instructions. Open the lid once fully depressurized.
Mix the cornstarch and cold water until smooth. Select Sauté and bring the cooking liquid to a simmer. Stir the slurry again, add it to the pot, and stir until the sauce thickens. Press Cancel.
Stir in the zucchini. Cover with a suitable nonpressurizing lid and let it cook in the hot sauce for approximately 4–5 minutes, until tender to your liking. Do not pressure cook again. If needed, briefly use Sauté while stirring to finish softening the zucchini.
Serve over separately cooked rice with optional toasted sesame seeds and sliced green onion.
