1Heat 2 tbs of oil in large saucepan over medium-high heat. Sauté celery root, onion, pears, and celery 2 minutes.
3Stir in 3 cups broth, cover pan, and bring to a simmer. Reduce heat to medium-low, and cook 20 minutes, or until vegetables are tender.
5Purée soup in batches in blender until very smooth, then return to saucepan.
7Add remaining 1/4 cup broth to blender, swirl to clean any soup from sides of jar and then pour into saucepan.
9Blend mustard greens, lemon juice, 2 tbs of oil and 1/4 cup soup in mini food processor until almost smooth.
11Season with salt and pepper, if desired.