Heat olive oil in a large saucepan over medium heat. Add leek, onion, fennel bulb, celery, and garlic halves; cover, reduce heat to low, and cook 45 minutes, stirring frequently. Add 2 cups water and remaining ingredients, and bring to a boil. Reduce heat, and simmer, uncovered, 1 hour. Strain mixture through a sieve into a bowl, and discard solids.
Note: Refrigerate the remaining Vegetable Nage in an airtight container for up to 1 week, or freeze it for up to 3 months.