Separate broccoli into small florets. Thinly slice mushrooms and onion.

In a pan, add broccoli and 100 ml water. Heat over medium heat until boiling, cover, and cook for 2 minutes. Remove broccoli and rinse the pan.

Heat butter in the pan. Once melted, add onion and sauté over medium-low heat until softened.

Add mushrooms and sauté until softened. Add broccoli and stir until coated in butter.

Add water and bring to a boil. Cover and simmer on low heat for 15 minutes. Let cool slightly.

Blend the mixture in a blender until smooth. Return to the pot, add milk and salt, and heat gently while stirring.