In a medium metal bowl, whisk together dry mustard and vinegar until smooth. Cover bowl and chill overnight.
Remove mustard-vinegar from refrigerator.
Fill a medium saucepan with 1-inch of water and bring to a simmer.
Meanwhile, to bowl of mustard-vinegar, add and whisk in cornstarch, sugar, egg, water, and salt. Set bowl over simmering water and cook mustard mixture 2 to 3 minutes, whisking constantly, until mustard thickens. Remove from heat, whisk for another minute, and allow mustard to cool. Homemade hot and tangy mustard is good for 2 weeks when covered and chilled.
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