Once the brown butter has cooled slightly, whisk in the mustard, honey, and vinegar and season with salt to taste. Whisk briefly, just until all the ingredients are evenly distributed. It won’t be emulsified and it won’t be homogenous — it will just be all mixed together, and that’s all you need. It is certainly possible that you may need to adjust the acid (vinegar) and the salt to find the best balance for you, so give it a taste, judge for yourself, and do what you need to do. Just be sure to stir or shake it before you dress something with it.