Remove the saucepan with the milk mixture from the heat, pluck out and discard the thyme sprigs, then pour the mixture over the potatoes (reserve the saucepan). Season the mixture with salt and pepper, then, using a hand mixer on low speed, mix until the liquid is incorporated, then increase the speed to medium and mix until just until the potatoes are light and smooth. (If you don’t have a hand mixer, you can just stir them vigorously with a whisk.) Taste the potatoes and season with more salt and pepper as needed, then smooth the surface into an even layer with a spatula.
