In a medium-size saucepan, bring three cups unsalted water to a boil. Add the garlic (or onion), potatoes, and bay leaf, and cook, partially covered, over moderate heat for 12 minutes of until the potatoes are fork-tender. Drain, reserving three tablespoons of cooking water, and discard the bay leaf.
Transfer the potatoes to a large mixing bowl. Add the margarine, buttermilk, and yogurt, and beat in with an electric mixer. Then, if needed, add just enough of the reserved water to make the potatoes fluffy.
Serves 4.