Add celery root, apples, and onion. Cook until apples and some of celery root are translucent (do not brown), stirring often, about 15 minutes.
Add 4 cups broth. Cover and bring to simmer. Reduce heat to medium-low; simmer covered until celery root and apples are soft, stirring occasionally, about 25 minutes. Remove from heat; cool slightly.
Working in batches, puree soup in blender until smooth, adding more broth by 1/4 cupfuls to thin to desired consistency. Return soup to pot.
Season to taste with salt and pepper. DO AHEAD: Can be made 1 day ahead. Refrigerate uncovered until cold, then cover and keep refrigerated.
Puree chives, grapeseed oil, and pinch of salt in blender until smooth.
Preheat oven to 375°F
Arrange pancetta slices in single layer on rimmed baking sheet. Roast until pancetta is browned and crispy, about 18 minutes. Transfer to paper towels to drain. Crumble pancetta.
DO AHEAD: Chive oil and pancetta can be made 2 hours ahead. Let stand at room temperature.
Rewarm soup over medium heat. Divide soup among bowls. Sprinkle pancetta crumbles over each serving. Drizzle each bowl with chive oil.