Steam broccoli, covered, about 2 minutes, or until tender. Place a Dutch oven coated with cooking spray over medium heat until hot.
Add onion, sauté 4 minutes. Add potato and next 8 ingredients (potato through broth); bring to a boil. Reduce heat; simmer, uncovered, 30 minutes or until tender.
Place 3 cups potato mixture in a blender; process until smooth. Add purée back to potato mixture in pan; stir well. Place flour in a bowl.
Gradually add milk; stir with a whisk until blended. Add to potato mixture. Cook over medium heat until thick (about 10 minutes), stirring constantly.
Stir in cheese. Add broccoli, crabmeat, lemon juice, and sherry cook until thoroughly heated.
Servings: 4 to 6