The primary ingredient in mustard is vinegar! And do you know what that means? The vinegar used to make a given mustard is HUGELY important in terms of the overall flavor and quality of the mustard.
1 cup ground yellow mustard seed
2/3 cup vinegar (Tangerine balsamic, Maple balsamic, Serrano Honey or Jalapeno (for heat), Oregano, Peach, Honey Ginger, Traditional...and on, and on.
1/3 cup honey (omit this if you are using a sweeter vinegar such as tangerine balsamic)
1 teaspoon sea salt, truffle salt, pink mineral salt, or your choice
Combine the ingredients in a small saucepan and cook over medium to medium-low heat for 6 minutes. Stir frequently until it thickens and then remove from heat. Allow the mustard to cool before storing it in an airtight container.
You can also play around by adding herbs, sea salts, ale, wine, chillies, peppercorns, horseradish, and on and on. The possibilities here are endless.
Depending on how spicy you like your mustard, you can refrigerate it at this point, which will preserve its fiery characteristic, or allow it to sit at room temperature for a day +/- which will mellow it's flavor the longer it sits out.
A tablespoon of this mustard is all you need to emulsify (hold together) a great vinaigrette or marinade. Place the mustard in a bowl, and slowly whisk in the vinegar of your choice. Whisking quickly and continuously, follow with the olive oil of your choice. The key here is to whisk briskly so that it creates a homogenous emulsification that holds the oil and vinegar together as opposed to what it would normally do which is separate.
Makes about 1 1/2 cups.