Preparation Briskly simmer milk, potatoes, garlic, thyme, salt, and pepper in a 2-quart saucepan over moderate heat, partially covered, 10 minutes (do not let boil). Add peas and cook, partially covered, until potatoes are tender, about 5 minutes. Remove from heat, then add butter and let stand, covered, until melted, about 1 minute. Discard thyme. Coarsely mash mixture with a potato masher or large fork. Thin with additional milk if desired.
Preparation Briskly simmer milk, potatoes, garlic, thyme, salt, and pepper in a 2-quart saucepan over moderate heat, partially covered, 10 minutes (do not let boil). Add peas and cook, partially covered, until potatoes are tender, about 5 minutes. Remove from heat, then add butter and let stand, covered, until melted, about 1 minute. Discard thyme. Coarsely mash mixture with a potato masher or large fork. Thin with additional milk if desired.