Lima Bean, Potato, and Garlic Purée with Potato Crisps

In a large saucepan combine potatoes, peeled and cut into 1-inch pieces, and garlic with enough water to cover by 1 inch and simmer, covered, 8 minutes. Add lima beans and simmer 8 minutes more, or until potatoes are tender. Reserve about 3/4 cup cooking liquid. Drain mixture in colander and force through a food mill fitted with a medium disk into a bowl. Stir in butter, salt and pepper to taste, and enough reserved liquid to reach desired consistency. Purée, covered and chilled, 1 day. Serve purée topped with potato crisps.