Mustard seeds for tempering lentils brought out a little bitter after taste I am fond of using whole spices for tempering therefore used mustard seeds along with cumin seeds, curry leaves, asafoetida &amp; nigella seeds. Thereafter used some cilantro and kasuri methi for finishing. What I noticed was that the lentils had a lil bitter after taste. Although previously I have used this spice blend minus mustard seeds a number of times without fail. What I know for sure is that I can take out mustard and make the tempering good again but somehow I want to use mustard seeds along. What else should I incorporate to bend mustard seeds in the spice mix to make it balanced?
There's quite some bitter components in there:


Fenugreek
Nigella seeds 
Mustard seeds


In general to balance bitterness, you either add salt, fat, or sugar. 

In yours case, maybe the addition of the mustard seeds pushed the bitterness just beyond what you like. So quite possibly you could also choose to reduce the amount of nigella seeds and fenugreek.