Place the potatoes and garlic in a large saucepan and cover with lightly salted water. Bring to a boil; cover and simmer about 20 minutes or until a fork easily pierces a potato.
Drain the potatoes and garlic. Mash with a potato masher or a fork until smooth. Blend in the milk, butter, parsley, scallions, Parmesan, and salt. Gently fold in the tomatoes.