Trim base and stalks from fennel bulbs, reserving a few leaves for garnish. Coarsely chop bulbs.
In a medium pan, combine chopped fennel, onion, butter, and 2 tbsp. water; cover and cook over medium-high heat until vegetables are limp, 5 to 7 minutes. Add pears and chicken broth; cover and simmer until pears are soft, about 5 minutes. Purée the mixture in a blender, in batches if necessary.
Return soup to pan to reheat. If it's too thick, add a little more broth. Season with salt and white pepper to taste. Ladle into bowls and sprinkle with fennel fronds and freshly ground black pepper.