How to Make It
Preheat oven to 450°F. In a large skillet, melt butter with 1 tablespoon olive oil over medium heat. Add onions and season with ½ teaspoon salt. Cook, stirring occasionally, until onions are golden brown and tender, 30 to 35 minutes.
Toss squash with 2 tablespoons oil and ½ teaspoon each salt and pepper on a rimmed baking sheet. Roast until tender and golden brown, 15 to 20 minutes.
Mix caramelized onions, squash, chestnuts, sage, and ½ teaspoon each salt pepper in a large bowl; set aside.
On a lightly floured surface, roll each piece of pizza dough into two long ovals about 18"x6" each. Divide squash mixture between ovals and arrange down the center, leaving a 1-inch border. Fold the left side of the dough over the squash and pinch it shut with the right side. Roll the log so it lays seam-side down. Repeat with the second oval. Twist logs together, pinching ends and tucking under to seal.
Transfer to a parchment-lined baking sheet (lay diagonally to fit) and brush with egg wash. Mix seeds and remaining 1 tablespoon oil; scatter seed mixture over twist and top with flaky salt. Bake until golden brown, 30-35 minutes. Let cool 5-10 minutes before slicing.
Make-ahead! To reheat, place in a 350°F oven for 20 to 25 minutes until crispy and heated through.