Make the butternut purée: Halve and seed a medium butternut squash and rub 1 tablespoon (14 grams) vegetable oil onto cut sides. Bake, cut side down, on a parchment paper-lined baking sheet at 375°F (190°C) until tender, 50 minutes to 1 hour. Let cool completely, cut side up, on pan. Scoop squash into the work bowl of a food processor, and pulse until almost smooth, stopping to scrape sides of bowl. (Alternatively, mash using a potato masher.) Reserve any leftover purée for another use.
Make the dough: In the bowl of a stand mixer, whisk together 1 cup (125 grams) flour, sugar, kosher salt, yeast, and onion powder by hand until combined.
In a small saucepan, heat ½ cup (120 grams) water, milk, and butter over medium heat, stirring occasionally, until butter is melted and an instant-read thermometer registers 120°F (49°C) to 130°F (54°C). Add warm milk mixture to flour mixture; using the paddle attachment, beat at medium speed for 1 minute, stopping to scrape sides of bowl. Add puréed butternut squash; beat just until combined, about 30 seconds. With mixer on low speed, gradually add 2½ cups (313 grams) flour, beating just until combined. Switch to the dough hook attachment. Beat at medium-low speed until a soft, somewhat sticky dough forms, 17 to 20 minutes, stopping to scrape sides of bowl and dough hook; add up to remaining ¼ cup (31 grams) flour, 1 tablespoon (8 grams) at a time, if dough is too sticky. (Dough will mostly pull away from sides of bowl.) Turn out dough onto a lightly floured surface, and shape into a ball.
Spray a large bowl with cooking spray. Place dough in bowl, turning to grease top. Cover and let rise in a warm, draft-free place (75°F/24°C) until doubled in size, 30 to 45 minutes.
Punch down dough; turn out onto a lightly floured surface. Divide dough into 8 portions (about 110 grams each), and shape into rounds. Cover and let stand for 10 minutes.
Lightly spray a 9-inch square baking pan with baking spray with flour. Line pan with parchment paper, letting excess extend over sides of pan.
On a lightly floured surface, roll or flatten each portion of dough into an 8x5-inch rectangle. Starting at one long side, roll each rectangle into a log, pinching seam to seal. Place, seam side down, in prepared pan. Cover and let rise in a warm, draft-free place (75°F/24°C) until puffed and fills pan, 30 to 45 minutes.
Preheat oven to 350°F (180°C).
Egg wash/Topping: In a small bowl, whisk together egg and 1 tablespoon (15 grams) water; brush onto dough. Sprinkle pepitas and sea salt evenly over the on top.
Bake until golden brown and an instant-read thermometer inserted in center registers at least 190°F (88°C), about 30 minutes. Let cool in pan on a wire rack for 10 minutes. Using excess parchment as handles, remove from pan. Serve warm.