Put the ginger and garlic into the container of an electric blender with 2-3 tablespoons of water and blend to a paste. Put the chops, tomatoes, onions, cayenne pepper, yoghurt, salt and ginger-garlic paste into a large wok or heavy-bottomed saucepan. Stir and bring to the boil.
Turn the heat to low, cover and simmer for 50 minutes or until the chops are almost cooked. Add the cumin seeds and simmer for 10 to 15 minutes or until the meat is tender and the sauce thick. Add the garam masala and lemon juice and stir. Sprinkle fresh coriander over the top and serve.
In the Punjab, tomatoes are grated to make a puree. Don't be tempted to use ready-prepared tomato puree instead of fresh. The spice combinations in garam masala vary in different parts of India. In the Punjab this one which you can try making yourself is common: 5 tbsps coriander seeds, 3 tbsps cumin seeds, 2 1/2 tbsps black peppercorns, 2 1/2 black cardamom seeds, 2 in cinnamon stick, 4-5 cloves, 1/6 nutmeg. Put the coriander and the cumin into a cast-iron frying-pan over a medium heat. Stir until lightly roasted. Allow to cool. Grind with the remaining ingredients in a clean coffee grinder and store in a tightly lidded jar.