Put the potatoes and garlic in a saucepan and cover with cold water by at least an inch. Tie the thyme, bay leaf, and lemon zest into a little bundle with kitchen string and drop it into thepan. Add a pinch of salt and bring to a boil. Reduce heat to medium, cover partway, and simmer for 10 minutes. Add the apples and continue cooking until the potatoes and apples are tender, another 15 minutes or so.
Drain well, reserving some of the cooking water. Discard the herb bundle and return the potatoes and apples to the pan. Set over medium heat and dry the potatoes and apples for a minute or two, shaking and stirring so they don't stick.
Turn the heat to very low and mash the potatoes and apples with a hand masher. Beat in the butter a few tablespoons at a time, with a wooden spoon. If the mash seems too dry, beat in some of the reserved cooking water until you get the consistency you like best. Often the apples are so juicy that you don't need to add any water. Season with salt and pepper and serve right away.
Recipe courtesy of One Potato, Two Potato (Houghton Mifflin, 2001) by Roy Finamore with Molly Stevens