Cut n separate each small cauliflower .
Then just blanched ( boil ) in hot water for 2 to 3 minutes .
Till get half boil .
White sauce :
In a separate sauce pan heat butter ,add flour mix well.
Remove from heat add water n milk .mix well
Now cook on low flame . Stir with whisk continuously to avoid lumps .
Add salt ,black pepper n mustard powder.
Cook till get slightly thick .
Add boiled cauliflower n mushroom slices in white sauce .
Remove from heat.