Coat a large saucepan with cooking spray and sauté onions over medium heat until tender, about 3 minutes. Add cauliflower and continue cooking for 2 more minutes. Add broth and milk to pan and bring to a boil. Reduce heat and simmer, covered, for 30 minutes or until cauliflower is very tender. Remove from heat; puree soup in blender (works best in batches). Return to pan over medium-low heat.
Whisk in cheese and continue to stir until melted.
Sprinkle with dill and serve.
Make it a meal by adding leftover chicken to the soup or try this as a side with vegetables for dinner!