1. Preheat the oven to 350°. In a medium skillet, combine the coriander, cumin, mustard and cardamom seeds and the cinnamon. Toast the spices over low heat until fragrant and starting to brown, about 1 minute. Transfer to a plate and let cool, then grind to a powder in a mortar or spice mill. Put the powder in a small bowl and stir in the turmeric and cayenne.
2. Put the rib racks on 2 large rimmed baking sheets, meaty side up. Rub the top of each rack with 1 teaspoon of the oil and season with salt and pepper. Spread the minced garlic and then the onion evenly on top. Sprinkle the racks with all but 2 teaspoons of the spice mixture. Roast the ribs for about 30 minutes, or until they start to brown.
3. Raise the oven temperature to 400° and sprinkle the racks with the reserved 2 teaspoons of spice mixture. Roast the ribs for about 30 minutes longer, or until the meat is tender. Cut the racks in between the ribs and serve hot or at room temperature.