For the curried mango sauce: Pour the oil into a medium-size saucepan over medium heat and, when it gets hot, add the garlic, ginger and onions. Cook until the onions are very soft and beginning to caramelize, 3 to 5 minutes. Add the garam masala, coriander, cumin, turmeric and cayenne pepper and stir as the spices toast and become aromatic, about 1 minute. Add the mango puree and stir well to combine. Season with salt and pepper and then let simmer for about 15 minutes. Keep warm.
For the crusted lamb rack: Preheat the oven to 400 degrees F. Oil a baking pan.
Mix the fenugreek, coriander, garam masala, fennel, paprika, salt and pepper together in a shallow dish. Then press the lamb rack into it, rolling it back and forth making sure it is well coated. Place in the prepared pan and roast until it reaches the desired level of doneness, 15 to 20 minutes for medium-rare. Let rest before slicing.
Serve the lamb with the mango sauce.