Place the green beans, chopped mushrooms, and onion flakes in the Instant Pot. Season with salt and black pepper, mix well then add in the ½ cup of water. Seal the lid, making sure to turn the valve to "sealing" and set the Instant Pot to "steam" for 2 minutes. (It will take 3-5 minutes to come to pressure.)
While the green beans cook, make a cornstarch slurry by mixing the cornstarch with the 2 tablespoons water. Set to the side, and mix just before pouring. At this time you will also want to toast the crispy fried onions for 1 minute in the oven until golden in color (make sure they don't burn).
Once the green beans are done cooking, carefully force the pressure out of the Instant Pot. Mix the green beans, then turn off the Instant Pot and then select "saute" mode. Next, make a well in the center of the Instant Pot.
Begin stirring in the cornstarch slurry followed by the heavy cream. Stir in the small hole you made until you see it starts to thicken then mix with the green beans. Remove from heat. Mix in ¼ of the crispy fried onions. Then top with the remaining. Enjoy immediately.