Place potatoes and garlic in a large saucepan; cover with cold water, and bring to a boil over high heat. Reduce heat; simmer, uncovered, until potatoes are tender when pierced with a knife (about 20 minutes).
Drain; return to pot over low heat.
Add milk, butter, salt, and pepper. Mash potatoes with a potato masher to desired consistency.
Preheat broiler.
Transfer potatoes to a shallow 1 1/2-quart ovenproof casserole or baking dish.
Combine cheese, breadcrumbs, and chives; sprinkle over potatoes. Broil 4-5 inches from heat source until cheese melts and starts to brown (3-4 minutes).
