How to Make It
Preheat the oven to 400°. On 2 large, rimmed baking sheets, drizzle the cut sides of the squash with oil and season with salt and pepper. Turn the squash halves cut sides down and roast for about 45 minutes, or until tender.
Meanwhile, in a large pot, heat 1 tablespoon of the oil. Add the mustard seeds and cook over moderately high heat, shaking the pot, until they pop, about 1 minute. Transfer the seeds to a small bowl. Add the remaining 1/3 cup of oil to the pot. Add the garlic and onion and cook over moderate heat, stirring occasionally, until softened, about 7 minutes. Add the coriander, turmeric and crushed red pepper and cook, stirring, until fragrant, about 1 minute. Remove from the heat.
Using a large spoon, scrape the squash flesh from the skins into the pot. Add the water and cook over moderately high heat, stirring and lightly mashing the squash, until blended and heated through. Season the squash with salt and pepper. Transfer to a bowl, top with the toasted mustard seeds and serve.