Rinse and pick over beans. In a medium saucepan, cover beans with water, and bring to a boil for 5 minutes. Transfer to a heat-resistant surface, and let beans stand in water, uncovered, 1 hour.
Drain beans; add fresh water to cover beans by 2 inches. Return to medium heat. Simmer, covered, until soft and tender, about 50 minutes.
Allow beans to cool slightly. Drain beans, reserving liquid. Place beans, garlic, olive oil, sage, lemon juice, and salt and pepper in a food processor; puree until smooth. If too thick, thin to desired consistency with reserved bean liquid. Transfer to a covered container; chill. To serve, brush bread with olive oil, and toast. Garnish spread with sage leaves or lemon slices, and serve the toasted crostini on the side.
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