Buttermilk in India means the real stuff - the liquid leftover from churning cream (well in India yoghurt or yoghurt cream) to make butter. Do not sub the American style cultured buttermilk. Instead dilute yoghurt with water until you get the right texture.
Seasoning wise there are many options. Start with salt. Then you may want to add minced ginger and chillies, raw. Other people prefer to add them to the tadka so they are cooked a bit. Heat some oil in a pan. When hot add black mustard seeds and hing, and when the seeds pop add curry leaves (and ginger and chillies if you want to add them here), let them sizzle for a moment or two and then add the mixture into the buttermilk. You can add chopped fresh coriander if you like too.
This is just one seasoning example. Some people do not do a tadka. Others add mint and coriander leaves. Some people use roasted spices, ground to a powder. And then of course some people like the buttermilk quite mild and others prefer it tarter so if the buttermilk itself doesn't taste right they add a little lemon juice. It all depends on region of India plus personal preferance.
Super; I had no idea I should dilute yoghurt to get that nice think texture. Now I remember they had green chilis & probably ginger in the drink, a few coriander leaves, & then the tadka as you suggest, That's exactly what I wanted; thank you so much!
especially for your lemon comment as I love mooru tart
Google "moru buttermilk" and you find many sites from Kerala,Tamilnadu.
Some sites I found: