Put the potatoes in a large pot of salted water. Bring to a low boil and cook until the potatoes are tender when pierced with a skewer, 15 to 20 minutes.
Meanwhile, in a medium saucepan, melt 1 Tbs. of the butter over medium-low heat. Add the garlic and cook, stirring frequently, until it has softened, 1 to 2 minutes. Add the milk, the remaining 10 Tbs. butter, and the lemon zest. When the butter has melted, season the mixture well with 1-1/2 to 1-3/4 tsp. salt. Keep the mixture warm while the potatoes continue cooking.
When the potatoes are done, drain them and return them to the pot in which they were cooked, set over low heat. Stir the potatoes to dry them until they just start to stick to the pot, about 30 seconds. Gradually add the warm milk mixture to the potatoes as you mash with a potato masher; if they’re dry, add up to 1/4 cup more milk. Season the potatoes to taste with salt and pepper and serve.(If you're working ahead, set the potatoes over a warm water bath, drizzle them with a few tabelspoons of milk, and cover loosely with foil. Just before serving, stir well.)