How to Make It
Select SAUTÉ setting on a programmable pressure multicooker (such as an Instant Pot). (Times, instructions, and settings may vary according to cooker brand and model.) Select HIGH temperature setting; allow cooker to preheat 3 minutes. While cooker preheats, sprinkle beef with pepper and 1 teaspoon salt. Add one-third of the beef and 2 teaspoons oil to cooker, and cook until meat is well browned on 2 sides, about 2 minutes and 30 seconds per side. Using tongs, remove browned meat to a plate; repeat 2 more times with remaining oil and meat.
Add mushrooms to cooker, and cook, stirring often, until browned, about 5 minutes. Transfer to a plate, and set aside. Add red onion, carrots, and garlic to cooker. Cook, stirring occasionally, until red onion begins to soften and lightly brown, about 3 minutes. Add red wine; cook until liquid has almost evaporated, about 9 minutes, stirring and scraping to loosen browned bits from bottom of cooker. Stir in beef, herb bundle, bay leaves, potatoes, 2 cups stock, and remaining 1/2 teaspoon salt; bring to a boil. Press CANCEL. Cover cooker with lid, and lock in place. Turn steam release handle to SEALING position. Select MANUAL/PRESSURE COOK setting. Select HIGH pressure for 30 minutes. (It will take 15 minutes for cooker to come up to pressure.) When program ends, carefully turn steam release handle to VENTING position, and let steam fully escape (the float valve will drop). (This will take about 2 to 4 minutes.)
Press CANCEL, and re-select SAUTÉ on HIGH temperature setting, and uncover cooker. Whisk together flour and remaining 1/2 cup stock in a small bowl; whisk into mixture in cooker until incorporated. Stir mushrooms, pearl onions, mustard, and vinegar into beef mixture. Cook, uncovered, until thickened, about 15 minutes. Turn off pressure cooker; discard herb bundle and bay leaves. Ladle stew into bowls, and sprinkle with chives.