Break the head of cauliflower into very small florets, discarding the thick stems.
You should have 4-5 cups of cauliflower florets, in the end.
Place the cauliflower florets in a large, deep sided saute pan.
Add the butter, water, cream ,salt, garlic powder and mustard.
Bring to a simmer over medium/high heat.
Cover and let cook, stirring occasionally, for 8-10 minutes, or until cauliflower is tender.
If the liquid is still thin, let it cook, uncovered, until it has reduced to the consistency of heavy cream.
Add 2/3 of the shredded cheese to the cauliflower and toss into the sauce until it melts and sauce becomes creamy.
Taste sauce and add more salt, if needed.
Remove from heat and sprinkle with the remaining cheese and a grating of fresh black pepper.
Assuming your pan is oven proof, set it under the broiler for 2-3 minutes to melt the cheese.
Serve immediately.