Put 1 cup of water in the Instant Pot. Add the steamer rack.
Put the cubed potatoes on the rack, followed by the whole head of cauliflower.
Set the Instant Pot to High Pressure for 4 minutes. Endure the valve on the lid is set to “Sealing”.
After the Instant Pot is complete, let it release pressure naturally for 10 minutes, then quick release the remainder of the pressure.
Purée the cauliflower (not the potatoes) in the food processor.
Mash the potatoes in a bowl.
Add the cauliflower purée to the potatoes and stir.
Add Greek yogurt, garlic, salt and pepper to mash and stir. Taste for seasonings, adjust and stir again.