Set cooker to saute-high.
Place both butter and oil in the cooker. Wait for 1 minute or until the butter is nearly melted.
Add chopped onions and minced garlic. Saute for 1 minute or until fragrant.
Press cancel on the cooker.
Add diced potatoes (½"x½"), salt, pepper, thyme, chicken broth and stir.
Place the lid on. Pressure cook on high for 8 minutes and quick release. Exercise caution when pressing the vent for quick pressure release. Follow the manufacture instructions for quick release. Remove the lid and press the cancel button.
Set the cooker back to saute-high.
Stir the soup with a ladle.
Combine cold milk and cornstarch. Stir until the cornstarch has completely dissolved.
Pour the heavy whipping cream and milk mixture into the cooker. Stir with the ladle.
Using a ladle and common spoon: scoop the cooked potato chunks up into the ladle and mash well with a spoon. Once satisfied, return the mashed potatoes back to the soup. Repeat 4-5 times for ideal consistency. (watch video prior to attempt)
Simmer for 5-6 minutes or until desired thickness. Make sure to stir frequently to prevent potatoes from sticking to the bottom of the cooker.