Blanch 1 cup cauliflower florets in medium pot of boiling salted water until just crisp-tender, about 2 minutes. Drain. Rinse under cold water; drain well. Set aside.
Melt butter in heavy large saucepan over medium heat. Add onion, leek, celery, and remaining cauliflower. Cover and cook until onion is tender but not brown, stirring frequently, about 8 minutes. Add flour and stir 2 minutes. Gradually stir in vegetable broth. Add 1 cup milk. Bring mixture to a boil. Reduce heat. Cover partially and simmer until vegetables are very tender and soup thickens, stirring occasionally, about 20 minutes.
Puree soup in blender in batches until smooth. Return to same saucepan. (Cauliflower and soup can be prepared 1 day ahead. Cover separately and refrigerate. Bring cauliflower to room temperature before using.)
Bring soup to simmer, thinning with more milk, if desired. Gradually add Stilton, stirring until melted. Season to taste with white pepper and salt.
Ladle soup into bowls. Garnish with reserved cauliflower and serve.