1 pound ground round beef
4 cups minced yellow onions (about 4 medium)
2 tablespoons olive oil
1 teaspoon sugar
8 ounces Cabot Sharp Cheddar, grated (about 2 cups)
1 teaspoon salt
½ teaspoon ground black pepper
3 sheets frozen puff pastry, thawed (from two 17.3-ounce packages)
1 large egg
1 tablespoon milk
COOK beef, in large skillet over medium-high heat, breaking it up with spoon, until well browned and no longer juicy; transfer to plate to cool.
ADD onions, oil and sugar to skillet and reduce heat to medium-low. Cook, stirring often, until onions are deeply golden and caramelized, about 25 minutes, reducing heat as needed. Let cool in skillet.
ADD cooled ground beef, cheese, salt and pepper to skillet and stir together well.
PREHEAT oven to 350°F.
ROLL puff pastry sheets lengthwise into larger rectangles, working with one sheet at a time on lightly floured surface, about 11 inches wide by 16 inches long. Using 5-inch bowl or plate as template, cut six 5-inch circles from each sheet. (Alternatively, cut each sheet into six 5-inch squares.)
PLACE about ¼ cup of filling in center of each circle. Whisk together egg and milk and brush some of this mixture on edge of circles. Fold pastry in half over filling, pressing with tines of fork to seal. (Turnovers can be made in advance up to this point; freeze until ready to bake.)
Place turnovers on baking sheets. Brush tops with additional egg mixture and prick top of each in several places with fork.
BAKE until puffed and golden, 20 to 25 minutes. Serve warm.