Place all the ingredients except the rosemary in a glass jar with a tight-fitting lid (such as a mason jar) and shake very well. Set aside in a cool, dark place and allow to sit for at least a day, until the seeds have soaked up the liquid entirely. Note: Be sure to use a non-reactive container, i.e not metal.
Purée the mixture to the desired consistency, using a little water when needed to get it going. I prefer the rustic look and texture of coarse grain. For a creamy mustard a little more water and time in the blender or food processor will be necessary. Note: Unless you have a highly efficient blender, it will be difficult to get your mustard as smooth as store-bought dijon.
When you’ve reached the desired consistency, stir two tablespoons of finely chopped rosemary into your mustard. This will keep in a sealed glass jar in the refrigerator for up to three months. Note: The mustard will become more mild as it sits, so if you find it too tangy, give it a week or two.