3/4 cup brown mustard seeds
3/4 cup yellow mustard seeds
1 cup white wine vinegar
1 cup dry white wine
1/4 cup extra-virgin olive oil
1/4 cup sugar
2 tablespoons dry mustard
1 tablespoon salt
1/2 teaspoon ground black pepper
1 teaspoon red pepper flakes (optional)
Put all ingredients into a deep, medium-sized bowl, stir well, cover and set aside at room temperature, stirring once or twice, for 24 hours.
Transfer contents of bowl to a food processor and purée until creamy. Transfer mustard to jars, seal tightly and refrigerate.