I had to smile when I read this question because I generally suffer from the opposite problem, namely, "How did all these mustards get into my fridge?"
At the moment I have half a dozen mustards: Dijon, honey, whole grain, tarragon, walnut, and one with chanterelles and shallots and chervil. It's like an addiction.
Indeed, I'd always assumed that having six or seven mustards clunking around in one's refrigerator door was a universal part of the human condition.
I'm relieved to learn that there are individuals out there who are free from this affliction.
Truly, I am. I'm also slightly intrigued. Who has mustard powder in the pantry but no mustard in the fridge? Not even a forgotten squeeze bottle of French's Yellow somewhere way in the back? Are you a minimalist? A compulsive fridge cleaner? It's hard for me to relate. Until very recently, I had condiments in my fridge that were older than my preschooler.
But enough about my fridge. To answer your question: Yes, you can substitute dry mustard for prepared mustard, but you're going to need to add liquid to it first.
In other words, you're going to make prepared mustard.
Mustard powder is ground up mustard seeds, and it's what gives prepared mustard its pungency (i.e. hotness). You could mix a teaspoon of mustard powder with two teaspoons of water to make a very simple mustard. It'll be hot and probably quite bitter.
You can do better.
If you have a recipe that calls for a tablespoon of prepared mustard, start with a teaspoon of mustard powder, add a teaspoon of water and a teaspoon of white wine vinegar (but any vinegar will do). Stir into a paste in a nonmetallic bowl and let it sit at room temperature for at least 30 minutes.
The acid in the vinegar helps to mellow out the heat of the mustard.
You could also add a teaspoon of sugar (or less, depending on your taste), or honey.
If you need more than a tablespoon, combine the following ingredients in a nonmetallic bowl:
- 4 teaspoon mustard powder
- 1 tablespoon water
- 1 tablespoon white wine vinegar
- 2 teaspoons honey
- 1/4 teaspoon salt
Stir into a paste and let it sit for about an hour. It'll make about 1/4 cup.
Note that while they will function in a pinch, these techniques are only mustard shortcuts. The best homemade mustard starts with whole mustard seeds. Here's a recipe for making homemade Dijon-style mustard.