It's certainly not turmeric. I use loads in BIR.
I have never got a taste like you suggest. I have tasted base by others which i can only describe as stewed onion.
Suggestions would be:
1) follow chewytikka 3 hr base
2) use more oil min 100 ml per 800g onion
3) cook very slowly
I currently cook stage 1a (onion, oil, salt, ghee) 1 hr, add spice and flavours (stage 1b) 30 min, blend then add water (stage 2) bring to simmer 20 mins. In past have cooked stage 1 6 hrs and stage 2 7 hrs with no bad effect.