Microwave the potatoes according to package directions.
While the potatoes are cooking, in a medium pot over medium heat, warm the cream with the butter, thyme, salt and pepper. Don't let it boil.
Put the potatoes into a large pot and mash with a potato masher. Strain half the cream mixture into the warm potatoes and add the cheese in chunks. Mix together, adding more cream to get a nice creamy consistency. Taste and adjust the seasoning with more salt, if necessary. Keep covered and serve warm.