Once the pancetta is cooked, set it to the side, add the onions and leeks to the pot, and cook. Season with a small amount of salt to draw out moisture and saute for 4 to 5 minutes.
Stir in the garlic and cook just until it is fragrant, which takes about 30 to 45 seconds. Next, add the celery, carrots, turnips, parsnips, and fennel and saute for 5 to 7 minutes over medium heat.