Pat the chuck roast dry, cut it into large chunks, and season generously with salt and pepper.
5 pounds beef chuck roast, salt and black pepper
Set the pressure cooker to Sauté. Sear the beef in batches until deeply browned on all sides, then transfer to a plate. Working in batches keeps the beef searing instead of steaming.
Add the garlic, ginger, brown sugar, miso paste, mirin, rice vinegar, dark soy sauce, and sesame oil to the pot. Stir until the miso dissolves and scrape up all the browned bits from the bottom.
5 cloves garlic, 2 inches fresh ginger, ¼ cup brown sugar, 2 tablespoons miso paste, 2 tablespoons mirin, 2 tablespoons rice vinegar, 2 tablespoons dark soy sauce, 1 tablespoon sesame oil
Return the beef and any juices to the pot. Lock the lid, set the valve to sealing, and pressure cook on high for 60 minutes.
Let the pressure release naturally, about 15 to 20 minutes, until the float valve drops.
Open the lid and skim the layer of oil off the top of the sauce.
Switch back to Sauté. Whisk the cornstarch with the water, stir it into the sauce, and simmer for 1 to 2 minutes until thick and glossy.
1 tablespoon cornstarch, 3 tablespoons water
Serve the beef and sauce over rice, topped with cucumber slices and green onions.
cucumber slices, green onions
