Bring lentils and chicken broth to a boil in a large saucepan over medium heat; cover, reduce heat, and simmer, stirring occasionally, 30 minutes or until lentils are tender. Drain lentils, if necessary.
Sauté chopped onion in hot vegetable oil in a large skillet over medium-high heat for 5 minutes or until onion is tender.
Process one-third of lentils, one-third of onion, and one-third of walnuts in a food processor, in 3 batches, until mixture is smooth, stopping to scrape down sides.
Stir together lentil mixture, salt, and pepper. Line a 3-cup mold with plastic wrap. Spoon lentil mixture into mold. Cover and chill 8 hours.
Unmold onto a serving plate. Garnish, if desired. Serve with celery sticks and crackers.
*Note: Beef or vegetable broth may be substituted for chicken broth.