First and foremost, roast the beetroot. This step is optional but I love roasted beets so I did it. Preheat oven to 425 F degrees. Wash and pat dry beets and wrap in aluminium foil. Place on baking tray and put it in the oven for around 20-25 minutes. They are done when you easily able to insert a knife inside. Once they cool down, peel and cut into cubes and set aside.
Take yogurt in a large bowl and whisk till smooth.
Add water to it and whisk till well combined. Add salt, black salt and chat masala. Set aside.
In a food processor, add chopped beetroot, ginger, curry leaves and green chili.
Grind everything together, without adding any water.
Transfer this mixture to the yogurt-water mix and mix till well combined. Do not strain this mixture right away.
Cover the bowl and put in the refrigerator for 1-2 hours, this gives time for all the flavors to mix well together.
After 2 hours, strain the mixture to get smooth buttermilk.
In a pan heat little oil, once hot add mustard seeds and let them top. Transfer seasoning to the prepared buttermilk and mix.