Combine onion, pear, garlic, ginger, mirin, soy sauce, and hot sauce (if using) in a food processor or blender and blend until smooth. Taste sauce; add salt and/or more heat, to taste.

In an Instant Pot/electric multicooker: Arrange short rib pieces in a single layer. Pour blended sauce over, and stir to coat. Secure the lid and set the pressure release valve to Sealing. Select the Meat/Stew setting for 35 minutes at high pressure. When the timer goes off, perform a quick release by moving the pressure release valve to Venting. Open the pot and add the carrots and daikon. Secure the lid on the pot again, and set the pressure release valve once again to Sealing. Select Cancel to reset the cooking program, then select the Manual program and set the cooking time for 3 minutes at high pressure. Perform another quick release when the time is up. Open the pot and use a slotted spoon to remove the meat and vegetables from the cooking liquid, then skim as much of the fat as possible from the surface of the sauce. Taste the sauce for seasoning and adjust as needed, then spoon the sauce over the meat and vegetables. Garnish with sesame seeds and scallions.

In the oven: Heat oven to 325 degrees F. Combine short rib pieces and blended sauce in a heavy casserole dish or Dutch oven. Cover tightly with foil and a tight-fitting lid. Bake in oven for 3 hours total, but after 2 hours, add the carrots and daikon. To check for doneness, carefully remove the lid and foil and pierce the meat; it should yield easily. Use a slotted spoon to remove the meat and vegetables from the cooking liquid, then skim as much of the fat as possible from the surface of the sauce. Taste the sauce for seasoning and adjust as needed, then spoon the sauce over the meat and vegetables. Garnish with sesame seeds and scallions.

To serve: Serve with white rice, a quick cabbage salad, and a quick cucumber salad, with gochujang on the side.

Do ahead: Short ribs are better the second and third days, and they keep in the fridge for 4 to 5 days.