Preheat the oven to 350 degrees. Line a baking tray with foil or baking paper.
Mix the yoghurt, curry paste and ginger together and spread thickly over the fat layer on the lamb. Place on the prepared tray and allow to stand for 10 minutes before cooking.
To make the sauce, cook the onion and cardamom pods in a good dash of oil in a saucepan over a moderate heat until the onion has softened; about 10 minutes. Stir in the curry paste and cinnamon stick, if using, and cook for a further 2 minutes or until very fragrant.
Add the water and lentils and simmer, uncovered, for 20 minutes until the lentils are tender (soften the flavor of the sauce with yoghurt or cream if wished)
While the lentils are cooking, place the lamb racks in the preheated oven to roast for 25 minutes.
Remove the lamb from the oven and set aside to rest for 5-7 minutes. Spoon the sauce into wide serving bowls. Carve the racks into thick cutlets each with two bones and sit them in curry sauce. Sprinkle with fried onion and chopped fresh coriander, and serve with your favorite Indian chutneys and breads.
Heat oil in a frying-pan and cook onion until crispy and slightly caramelized. Transfer to a paper towel-lined plate to soak up the excess oil. Serve when cool and crispy.