In a blender, puree half of the diced onion with the coriander and cumin seeds, and 1/4 cup of the water.

In a large skillet, heat the vegetable oil. Add the remaining diced onion and the jalapeno slices. Cook the vegetables over moderately high heat, stirring a few times, until the onion begins to brown, about 5 minutes. Scrape in the onion puree from the blender. Pour 1/4 cup of the water into the blender; swirl it around, then pour it into the skillet. Cook over moderate heat, stirring occasionally, until the onion sauce begins to stick to the bottom of the skillet and turn brown, about 10 minutes.

Stir in the remaining 1/2 cup of water, scraping up the browned bits from the bottom of the skillet. Stir in the cauliflower and season with salt. Cover and cook over moderately low heat, stirring occasionally, until the cauliflower is tender, about 30 minutes. Stir in the diced tomato and the minced cilantro and cook until warmed through, about 1 minute. Serve.