Mix the dry ingredients together in a small bowl until evenly combined. Add water and stir well until a liquid paste forms and all dry ingredients are absorbed. Next, add oil and vinegar and stir well until evenly combined.
Let your Chinese hot mustard rest for 10 minutes covered, and re-stir to ensure the dry ingredients have fully absorbed. At this point, taste your Chinese Hot Mustard and adjust it to your own preferences.
Add a little more water or oil if you like a thinner in consistency. Add more vinegar if you like it a tad tart. Omit the vinegar altogether if you like it spicier, since vinegar makes your mustard a bit mellower in flavor. Add more white pepper and/or mustard powder if you like it spicier.
NOTE: Since Chinese mustard is so easy to make, we like to make in small amounts to have it fresh every time. Feel free to multiply the ingredients proportionally to make larger batches.