Turn the Instant Pot on to “sauté“ mode. Place 1 tablespoon of olive oil in the Instant Pot, and add the diced onions and minced garlic. Let cook for one minute then add the container of sliced baby Bella mushrooms. Sauté and cook for 5 minutes.
While the mushrooms are cooking, season the turkey tenderloins with the ground sage, garlic powder, salt, black pepper on both sides. Move the the mushrooms, onions, garlic to the side and place 1 tablespoon of olive oil on the bottom of the pot. Place the turkey tenderloins in the Instant Pot and sear for two minutes on each side until a light golden crust begins (about 3 minutes.
Remove the turkey tenderloins from the Instant Pot, and deglaze with 1/2 cup of chicken stock. Stir the mushrooms and onions around making sure to remove all the brown bits from the bottom of the Instant Pot. Once again, move the mushroom mixture to the sides, and add the turkey tenderloins back into the Instant Pot.
Close the lid, moving the pressure valve to “sealing“ and set on “meat/stew“ setting for 6 minutes. Once the Instant Pot is done cooking let it rest for an additional 5 minutes. Carefully force the pressure out of the Instant Pot. Remove the turkey tenderloins from the Instant Pot, turn it off and then on again and select "sauté" mode.
Create a cornstarch slurry in a bowl mixing both cornstarch and 1/4 cup chicken stock. Begin to stir the cornstarch slurry into the Instant Pot, and mix until the sauce thickens. Add in half cup of heavy cream, stir well, and turn the Instant Pot off.
Slice the turkey tenderloins, place back in the Instant Pot. Coat the turkey with the creamy mushroom sauce, then divide evenly among plates. Ladle additional mushroom cream sauce on top.