Peel and slice turnips, boil/steam in a small pot or saucepan until tender. (A fork should be able to pass through without resistance.)
Drain excess water from turnips (if any) and add Desi ghee (or coconut oil, if substituting.) Mash with fork or potato masher until ghee is well incorporated and turnips have no lumps.
In a separate pot, heat oil. Add the cumin and wait for it to start sizzling/crackling then add the onions.
Fry onions on medium heat until they turn a light golden brown.
Reduce flame and add ginger paste and garlic paste. Fry until masala is golden brown and garlic no longer gives off a "raw" smell.
Add salt, red chilli powder, and turmeric powder. Fry until the "raw" scent of the turmeric goes away. Then add the coriander powder and fry for a few seconds.
Add the mashed turnips to the pot and increase heat to medium. Fry until incorporated and excess water dries up.
Add the chopped tomatoes and fry until they start to break down.
Decrease flame to a simmer, cover and cook, stirring occasionally until the tomatoes have completely broken down and the curry is homogeneous.
Add black pepper powder, chopped green chilli and chopped coriander leaves. Mix to incorporate.
(Optional) Garnish with coriander leaves and serve with yogurt.
If you're confused about all of this talk about the "raw" smell of certain ingredients and spices, I explain more about this in my Pakistani Home Cooking Guide.