Crispy Butternut Squash Samsa

Name: Crispy Butternut Squash Samsa
Ingredients: 1 (14- to 17-ounce) package frozen puff pastry (preferably all butter), thawed in the refrigerator; 3/4 teaspoon cumin seeds; 1 pound honeynut or butternut squash; 1 medium yellow or sweet onion; 1 1/2 teaspoons kosher salt; 1 teaspoon freshly ground black pepper; 2 teaspoons all-purpose flour, divided, plus more for dusting; 4 teaspoons ghee or unsalted butter, divided; 1 large egg; 1 tablespoon water; 2 teaspoons nigella seeds; Flaky salt, for topping (optional)