Now getting into Bengali food from History of it I think the most common question that my friends pose in context of bengali food is regarding mustard seed and paste. It is impossible to think of bengali food without thinking of mustard, especially the famous Shorshe Mach (Fish in mustard sauce). The dish itself has quite a few varieties and each is a delicacy in itself. Coming back to mustard, there are three types of mustard seeds–
White mustard: these seeds provide the mildest flavor.

Brown mustard: these seeds are more pungent than white
Black mustard: black seeds have a very strong and distinctive flavor
Mustard is used in many forms and in a whole lot of recipes. The whole Mustard seeds are used for tempering, to add flavor to the oil. The Mustard paste is very popular in mostly Bengali or east Indian cooking. Mustard Oil is also an important part of Bengali Cooking and traditional Bengali Cooking mostly used Mustard Oil as the cooking medium. There is also a spicy, tangy variety of table mustard called 'Kasundi' that is used as a sauce.
For the sorshe mach, I use a mixture of black and yellow mustard, they kind of balance each other out. About equal parts of each. Grind mustard to a paste with green chillis and salt, this prevents the mustard paste from turning bitter. Though my mother swears by the traditional Shil-nora (mortar and pestle), I am quite happy with my humble mixer and grinder. The problem is that one cannot grind small quantities of mustard in that mean machine. The easier option is to grind about a cup of mustard, with one tbsp of salt and about 6 green chilles, use the amount required and store the rest in an air tight container in the freezer. The paste stays put for about two-three weeks.
Now that we have the mustard paste out of the way, may be I can gather courage to proceed to some recipes using this paste...
