Add mustard seeds, green chilli, and ¼ teaspoon salt in a stone mortar and pestle. Grind to a coarse paste. Or you can use a tiny blender as well for the purpose. Transfer the paste to a bowl and set it aside.
Peel and cut the cucumber lengthwise into half. Remove the seeds from each piece using a spoon. Grate the cucumber, and squeeze out all the liquid from the grated cucumber using a muslin cloth. Set aside in a bowl.
Whisk curd to get a smooth consistency. Use approximately ¼ cup of water to get the desired consistency. Kumaoni Kheere ka raita is usually on the thicker side.
Add the mustard paste to the whisked curd, grated cucumber, mustard oil, coriander leaves, remaining salt, and turmeric. Mix nicely. Taste, and if required, add more salt or mustard oil.
For the flavours to mature, let the raita sit at room temperature or in the refrigerator for 30 minutes before serving.
Serve Kumaoni Raita chilled with aloo ke gutke.