1. In 2-quart saucepan bring 1 1/2 quarts water to a boil; add potatoes, onion, and garlic and cook until potatoes are fork-tender, 10 to 15 minutes.
2. While potatoes are cooking prepare milk mixture. In small nonstick saucepan combine milk, margarine, and pepper and cook over low heat until margarine is melted. Keep warm over low heat.
3. Pour potato-onion mixture through colander, discarding cooking liquid. Transfer potato-onion mixture to large mixing bowl. Using mixer on low speed, mash potato-onion mixture. Gradually increase speed to high; add milk mixture and continue beating until potatoes are light and fluffy.