1/2 cup Colman’s dry mustard
1/2 cup white wine vinegar
1 large egg
1/4 cup sugar
1 teaspoon cornstarch
1/2 teaspoon salt
Whisk together dry mustard and vinegar in a medium metal bowl until smooth. Chill, covered, overnight.
Bring a medium saucepan filled with 1 inch water to a simmer. To bowl of vinegary mustard, whisk in 1/4 cup water, egg, sugar, cornstarch, and salt. Set bowl over simmering water and cook, whisking constantly, until mustard thickens, 2 to 3 minutes. Remove from heat, whisk for a minute, and then let cool.
Make ahead: 2 weeks, covered and chilled.