Step 1: In a small bowl, combine the dry ground mustard powder and 3 tablespoons of cold water. Stir vigorously with a spoon or whisk until a thick paste forms. If it's too thick, add the remaining tablespoon of water gradually until you reach a consistency similar to thick paint.
Step 2: Cover the bowl tightly with plastic wrap and let it sit at room temperature for at least 10-15 minutes. This crucial step allows the enzymes in the mustard powder to react with the water, releasing the volatile, pungent oils that create the signature heat of chinese hot mustard sauce.
Step 3: After resting, stir in the neutral oil, white vinegar (if using), and salt (if using). The oil helps to mellow the initial harshness slightly and adds a smooth texture, while vinegar adds a touch of brightness.
Step 4: Taste and adjust. If you prefer more heat, you can add a tiny pinch more mustard powder and a few drops of water, then let it rest again briefly. If it's too thick, add a few more drops of water. If too thin, a tiny bit more mustard powder.
Step 5: Your homemade chinese hot mustard sauce is now ready to serve! Transfer to an airtight container or a small jar. For best flavor and potency, serve immediately or within a few hours. It will gradually lose some of its punch over time.