Put the coriander, cumin, nigella, and cardamom seeds along with the peppercorns in a dry nonstick skillet. Place over low heat and gently warm them through for about 5 minutes until fragrant and toasty. Shut off the heat and add in the ground spices, tossing them for just a few seconds before tipping into a spice grinder. Grind to a fine powder.
Stir the yogurt, milk, and rose water—be very careful not to overdo it with the rose water—and set in the fridge.
Preheat the oven to 425°
Peel and scoop the seeds from the squash. Cut the squash into ½” x 2” pieces . Toss them with the oil and spices and spread over a baking sheet in a single layer. Roast for about 20 minutes, turning halfway through. Sprinkle with salt and roast another 10 minutes or so before transferring to a serving platter—they should obviously be soft but not total mush, but that said if they need a few minutes longer let them have it.
Drizzle the yogurt sauce over top before scattering with the cilantro and pom seeds.
The vegetables are still very good at room temperature so if it helps you can roast them in advance—say up to an hour before serving—and just keep them on the counter while you get on with everything else. You can also keep them warm in the shut-off oven once the turkey comes out (if you're making it for Thanksgiving, of course). Just be sure to hold back the sauce, cilantro, and poms until just before serving.