What spice can I use in place of brown mustard seeds? Or should I just leave it out?
Unfortunately There really is no substitute for the mustard in the tempering, except perhaps the whole white mustard seeds. (the mustard heats up to pop and release the flavor laden oils). If you don't have access to any kind of whole mustard , just omit the it altogether.
Attempt to find some if you can in your area. They are brilliant and give a wonderful flavor. But yes, no substitute. :(
I use whatever color I have around. If you can find an Indian grocery store you can get a huge bag of mustard seeds for a few dollars, also the other spices in this recipe, so you will NEVER need to omit them.