Heat oven to 425 degrees. Brush squash with 2 tablespoons oil and season with salt and pepper. Roast cut side down on two baking sheets until tender and caramelized, 15 to 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 oil, and vinegar. Season with salt and red-pepper flakes. Divide filling among squash.
Everything about this recipe is perfect! It's protein packed and healthy, the presentation is beautiful and perfect for dinner parties and as for left overs- fry it up with a light amount of coconut oil in a skillet and its perfect the next day! I will absolutely be making this again!