In the Instant Pot, combine the beef, kimchi, water, onions, mushrooms, garlic, ginger, sesame oil, soy sauce, gochugaru, gochujang, and sugar. Secure the lid on the pot. Close the pressure-release valve. Select MANUAL or PRESSURE COOK and set the pot at HIGH pressure for 15 minutes. At the end of the cooking time, allow the pot to sit undisturbed until the pressure has released. Taste and season with salt. Stir in the scallions and the tofu, if desired, and serve.