Mustard seeds are fairly high fat and create an emulsion (like mayo for you), and they have this lovely mustardy flavor that doesn't have to be spicy. It can be smooth and creamy and just the tiniest bit astringent.
It can be used as a vehicle (as mayo often is), for spice, or tailored to be hot on its own. I use it to make up a buffalo hot sauce on occasion.
For more richness, tahini can be added, and for more tangy-ness more vinegar. Anything high in salt, sugar, or spice can overwhelm the gentle flavor of mustard flavor itself deadening the taste buds to the mustard flavor.