Deeply savory from the caramelized squash with bites of saltiness from the feta, nuttiness from the pistachios, and freshness from the parsley. The salt, pepper, olive oil, and vinegar round off the flavors
Preheat oven to 425 degrees F. Brush squash with 2 tablespoons olive oil and season with salt and pepper. Roast cut-side down on two baking sheets until tender and caramelized, 15-20 minutes.
Meanwhile, bring quinoa and 2 cups water to a boil in a small pot. Reduce heat and simmer, covered, until tender and water is absorbed, about 15 minutes. Let cool, then fluff with a fork. In a large bowl combine quinoa, parsley, feta, pistachios, remaining 2 tablespoons olive oil, and vinegar. Season with salt and red pepper flakes to taste. Divide filling among squash.