Heat 1 tablespoon oil in a heavy-bottomed saucepan over low heat until shimmering. Add 1 tablespoon chopped onion, coriander seeds, cumin seeds, dried chili, peppercorns, cloves, ginger, and garlic. Cook, stirring frequently, until fragrant, about 5 minutes. Turn off the heat and add fennel seeds. Stir to mix through. Immediately transfer to the bowl of a blender or mortar and pestle and blend/pound until a fine paste is formed, scraping down sides as necessary.
Wipe out saucepan with a paper towel and add remaining oil. Heat over medium heat until shimmering. Add the remaining onion. Cook, stirring, unti lightly browned, about 6 minutes. Add the tomato puree, bring to a simmer, and simmer for 5 minutes. Add turmeric powder and the ground spice paste. Stir well to combine and continue to cook, stirring occasionally, until the oil separates and starts surfacing. Add a few drops of water if the paste tends to stick or get too dry.
Add the crab and salt. Spoon some of the sauce over the crab to coat. Cover and cook for 5 minute. Remove lid, stir, and continue to cook for 10 minutes, spooning the sauce over the crabs occasionally. Sprinkle with coriander leaves and serve immediately with rice or bread.