Add potatoes then milk directly into the Instant Pot, then add the spices, garlic, and butter - making sure the butter is evenly spread out. (With the garlic, remember that we measure garlic with our hearts, so if you are a huge garlic lover, add extra.)
Stir everything together and try to make sure the potatoes are pushed under the milk as best they can be and the butter and spices are evenly spread around.
Close the lid of the Instant Pot and point the vent to Sealing. Then press the Manual or Pressure Cook button followed by the Pressure button to adjust it to LOW pressure. Set the time using the +/- buttons or the dial for 12 minutes.
When the cooking cycle is complete, carefully release the pressure using the "Quick Release" method - turn the vent to Venting and let the steam out right away. You can either turn the Instant Pot Off or leave it on the Keep Warm setting.
Once all pressure has been released, carefully open the lid.Drain the potatoes into a colander over a bowl, but save the milk to add back later. Mash the potatoes using your favorite method - a ricer, masher, or pastry cutter in times of desperation - I like to do this right in the Instant Pot.
Add half of the milk back and stir. If it still seems too thick, continue adding more milk until you reach your desired consistency. Season the potatoes with a pinch of additional salt, pepper, and thyme and taste. (You can add more butter if you like, but I like to add it when serving.)
If adding cheese, stir it in, set the Instant Pot to the Keep Warm function, and let cook, stirring occasionally, until the cheese is melted and creamy.
You can keep the potatoes covered with a glass lid in the Instant Pot, on the Keep Warm setting, for up to 4 hours.
When ready, serve the potatoes topped with additional salt, pepper, thyme, and butter.