In a kadai, heat mustard oil. Add the potatoes, fry the potatoes and keep them aside.Meanwhile, mix turmeric powder, cumin powder, coriander powder and red chili powder. Add ¼ cup of water to the mixture.Temper the oil with whole red chilies, bay leaves and cumin seeds. As the oil turns aromatic add the grated ginger, followed by asafoetida and the spice mixture.Add the tomatoes, potatoes, coconut and salt and continue cooking on medium flame.Cook till you see oil leaving the sides of the pan.Add water to the kadai and let it simmer.Gently add the dhokas to the kadai and continue cooking till you achieve the desired consistency. (See Notes)Add the sugar, Bengali garam masala powder, ghee and let it simmer for 2 minutes.Turn off the flame and serve the dhokar dalna with steamed rice, Bengali fried rice or basanti polao.