Melt butter in large saucepan over medium heat. Add celery, onion, and potato; cook about 4 min., until vegetables begin to soften. Add broth, 2 cups (500 mL) water, salt and pepper. Bring just to a boil over high heat then reduce to a simmer 10 to 15 min., or until potato is soft. Add broccoli. Simmer an additional 5 to 7 min., or until broccoli is fork-tender.
Use an immersion blender to purée soup until smooth. Divide soup into 8 serving bowls and top each with 1 tbsp (15 mL) shredded Cheddar cheese before serving.
To make a curried broccoli soup, add 1 tbsp (15 mL) curry paste when cooking the celery, onion and potato.