To prepare croutons, combine cumin and 1 teaspoon oil in a bowl; add bread cubes, and toss to coat. Spread bread cubes on a baking sheet. Bake at 350° for 10 minutes or until golden. Cool croutons slightly; set aside.
To prepare soup, heat 2 teaspoons oil in a large saucepan over medium heat. Add shallots and celery; cover and cook 2 minutes. Stir in cauliflower, potato, broth, salt, and pepper; bring to a boil. Reduce heat, cover, and simmer 15 to 20 minutes or until vegetables are tender. Add lemon juice. Place vegetable mixture, in batches, in a food processor; process until smooth. Divide soup evenly among 4 bowls; serve with croutons, and top with chives, if desired.