Martha's recipe was inspired by a potato puree from the restaurant Eleven Madison Park.
Fill a large pot with about 2 inches water. Set steamer basket in pot (water should not come through the holes). Bring to a boil, and then reduce to a rapid simmer.
Add potatoes to steamer basket and steam until tender when pierced with a knife, 25 to 30 minutes.
Meanwhile, melt the butter in a medium saucepan over medium heat. Simmer, whisking occasionally, until the butter is deep golden brown, 5 to 10 minutes. Place the thyme and garlic in a fine mesh sieve lined with cheesecloth. Strain the butter over the garlic and thyme into a medium bowl, add cream, and whisk to combine.
Remove potatoes from steamer basket. Drain, and immediately press through a medium mesh sieve into a large bowl using a rubber spatula. Fold the cream mixture into the potatoes. Press mixture through a fine mesh sieve. Season with salt and pepper and serve immediately.