Roasted Winter Squash with Yogurt and Pistachios

1 winter squash (about 2 pounds), preferably kabocha, red kuri or acorn
2 tablespoons extra-virgin olive oil
1 teaspoon kosher salt, or more as needed
1/2 teaspoon freshly ground black pepper
4 tablespoons unsalted butter
1/2 cup raw shelled pistachios, coarsely chopped
1/2 teaspoon ground cumin
1/2 teaspoon ground turmeric
1 pinch ground cinnamon
1 to 2 pinches crushed red pepper flakes (optional)
1 cup Greek yogurt
2 tablespoons fresh lemon juice

Preheat the oven to 425 degrees with the rack in the middle.

Using a large, sharp knife, stab the squash in the middle and carefully work the knife down to cut the squash in half, going through the stem. Scoop out the seeds and stringy pulp, and discard or reserve for another use. Cut the squash halves into 1 1/2-inch wedges, leaving the peel on.

On a large, rimmed baking sheet, drizzle the squash with the olive oil. Season with 1/2 teaspoon salt and the pepper and roast for 40 to 50 minutes, until the squash is very tender and golden brown with caramelized bits.

Meanwhile, in a small pot over medium heat, melt the butter. Cook, swirling occasionally, until the butter has browned and started to foam, about 5 minutes. Remove from the heat and add the pistachios, cumin, turmeric, cinnamon and pinch of red pepper flakes, if using. Stir in 1/4 teaspoon salt, taste, and add more salt, if needed.

In a small bowl, whisk together the yogurt, lemon juice and the remaining 1/4 teaspoon kosher salt. Taste and add more salt, if needed. Spread the yogurt sauce over a large serving platter or bowl. Arrange the squash on the sauce and spoon the buttered pistachios over everything. Sprinkle with another pinch of red pepper flakes, if desired, and serve.