Cube your steak and slice your mushrooms.
Place steak and mushrooms in the bottom of your Instant Pot liner. Season with salt and pepper. Pour 1 1/2 c. broth over top. Close lid, turn knob to 'seal' and set to 'manual' for 5 minutes.
When the timer goes off, quick release the pressure. Remove the lid. Remove 1/2 c. of liquid and set aside.
Turn pot to 'saute' and add butter. Stir until it melts. Add sour cream and cream cheese and stir until melted and smooth.
Add cornstarch mixed with water and stir. Let mixture bubble, stirring occasionally, until thickened to desired consistency. If it is too thick, add reserved broth in, just a couple of tablespoons at a time. Season with salt and pepper to taste.
Serve beef stroganoff over cooked noodles or mashed potatoes.
Leftovers: Store leftovers in the fridge. Use within 3-4 days.