Melt butter in a Dutch oven over medium heat. Add leeks, celery, and garlic; sauté 10 minutes or until vegetables are tender, but not brown. Add cauliflower; sauté 2 minutes. Add 6 cups broth; bring to a boil. Reduce heat, and simmer, uncovered, 20 minutes, or until vegetables are very tender. Cool slightly.
Puree vegetables, in batches, in a blender or food processor until very smooth and creamy. Return pureed vegetables to pan. Stir in half-and-half, salt, and pepper. Bring soup to a simmer over medium heat. Stir in remaining 3/4 cup broth, and cook 5 minutes or until thoroughly heated. Garnish, if desired.