Set instant pot/pressure cooker to saute. Salt and pepper the meat generously all over. Drizzle oil in the pot and add meat chunks, allowing to brown for 2-3 minutes, then flip over and sear on the other side for 2-3 minutes. Sprinkle onion soup mix over the meat, add potatoes and broth.
Cover, make sure vent is set to sealed position, and set to PRESSURE COOK or MANUAL for 60 minutes. Do a natural release for 15-20 minutes (don’t do anything, just let it naturally depressurize) then quick release (turn valve to VENT) and remove lid once float valve drops down.
Transfer beef to a serving platter and shred with two forks. Use a strainer to remove any fat or large bits from the liquid in the pressure cooker. Set to SOUP setting and move on with the next step.
Transfer potatoes to a large bowl. Add sour cream, salt, pepper, and garlic and hand mash or use an electric hand mixer until creamy and smooth.
By now the liquid in the pot should be boiling. Whisk together cold water and corn starch and stir into the boiling liquid. Season with salt and pepper (taste, add more if needed) and serve over mashed potatoes.
Follow instructions above, but do not cut meat into sections (since it’s frozen and very hard to cut through) and skip the searing step. Pressure cook for 90 minutes.