Roast mustard seeds, cloves, fennel seeds and whole dry red chillies. Grind to a fine powder and add turmeric powder, nigella seeds and red chilli powder to make the spice blend for the dish.
Heat half of the mustard oil in a kadai/deep bottom pan and sauté ginger-garlic paste. Add the ground spice blend and sauté for a minute on low heat.
Add the lamb cutlets, mix well and brown for 5 minutes. Remove the cutlets and keep aside.
In the same pan, heat the remaining oil and add chopped onions and sauté till golden brown.
Add tomatoes and sauté again till the tomatoes completely break down and turn mushy; oil starts clearing at the sides of the caramelized onion tomato mixture.
Return the lamb cutlets to the pan and add enough water to cook the lamb.
Season with salt.
Cook for 5 minutes on high heat and then slow cook till the lamb/mutton is soft and tender.
Garnish with coriander leaves.