Roasted Winter Squash and Onion Turnovers

butternut squash, halved lengthwise and seeded

Salt and freshly ground pepper

medium sweet onion, sliced 1/2 inch thick

shredded Gruyère cheese (5 ounces)

finely chopped thyme

All-purpose flour, for rolling out the pastry

Flaky Turnover Pastry

Egg wash made wth 2 large egg yolks lightly beaten with 2 tablespoons whole milk