Cook whole cauliflower as directed or break into flowerets and cook as directed
Drain cauliflower thoroughly; keep warm.
Meanwhile, cook fresh mushrooms in butter till tender, about 4 minutes.
Blend flour, white pepper, and 1/4 tea spoon salt into butter.
Add milk all at once.
Cook and stir till thickened and bubbly.
Stir in cheese and mustard.
If using canned mushrooms, stir them into sauce.
Heat till cheese melts.
Place head of cauliflower on platter; spoon some sauce over.
Pass remaining sauce or, pour sauce over flowerets.
Sprinkle with parsley.