Making your own mustard isn't hard, and you can customize your flavors.
Mix all ingredients in a small bowl. Cover and let sit soak for 2-3 days or until mustard seeds have softened. Place all ingredients in a blender. Blend until creamy adding water as needed (2 tablespoons at a time)
Note: Fresh or dried herbs, dried fruits, peppers, etc.
Date & Walnut: Add 3 pitted dates to mustard seeds. Process until creamy. Fold in 1/4 cup finely chopped walnuts. YUM!!!
Sources: Once Upon a Tart, Tales of a Kitchen and Homemade Collective