Step 1Heat the oven to 425 degrees.
Step 2Cut the squash in quarters. Scoop out the seeds and place the squash pieces cut-side up on a baking pan. Spray the squash with nonstick cooking spray. Combine the coriander, cumin seeds and 1/2 teaspoon of salt in a spice grinder; grind to a coarse texture. Sprinkle half the spice mixture over the squash.
Step 3Roast the squash until it is tender and browned around the edges, about 1 hour.
Step 4Combine the yogurt, sour cream, zucchini, garlic, the remaining 1/2 teaspoon of salt and pepper to taste. Spoon the tzatziki sauce over the roasted squash. Sprinkle with the remaining spice mixture.