Instant Pot Buttery Garlic Mashed Potatoes

 lbs (1.5kg) red and Yukon Gold potatoes, peeled if desired, halved or quartered

garlic cloves, peeled

 1/2 cups (375ml) low sodium chicken stock

butter or ghee, plus more for serving

 cup heavy cream

Salt and freshly ground black pepper

Minced fresh thyme, for garnish (optional)