1/4 cup Colman’s dry mustard
1/2 cup white wine vinegar
1/4 cup agave nectar
2 teaspoons canola oil
2 tablespoons black or brown mustard seeds
1 large egg
1 teaspoon salt
2 1/2 teaspoons cornstarch
Stir together dry mustard, vinegar, 2 tbsp. water, and agave nectar in a medium metal bowl until smooth. Chill, covered, overnight.
Put oil and mustard seeds in a small frying pan and heat over medium heat, covered. As soon as mustard seeds start to pop, about 3 minutes, remove from heat. Let cool.
Bring a medium saucepan filled with 1 inch of water to a simmer. To mustard-vinegar mixture, add toasted mustard seeds in oil, egg, salt, and cornstarch and whisk to blend. Set bowl over saucepan and cook, whisking constantly, until mustard thickens, 3 minutes.
Make ahead: 2 weeks, covered and chilled.