Gather all the ingredients listed in your shopping list. Make sure your boiling water is ready, as it will help activate the mustard’s flavor.
In a mixing bowl, combine the dry mustard and salt. Stir them together to ensure even distribution.
Carefully pour the boiling water into the bowl with the mustard and salt mixture. Use a whisk or fork to mix until it's smooth. You will notice the mixture thickening as you stir.
Once the mixture is smooth, add the vegetable oil. This will give your mustard a silky texture and enhance its flavor. Stir well to combine.
Taste your mustard. If you’d like more heat, you can add a bit more dry mustard. If it’s too hot, a touch more oil can mellow it out.
Allow your homemade mustard to sit for about 10 minutes. This resting time will deepen the flavors and allow the heat to develop fully.
Transfer your mustard to a sealed container and refrigerate. It’s ready to use, but it will be even better after a day in the fridge as the flavors continue to meld.