1. Heat oven to 400 degrees F. In a bowl, stir together cheese, ham, half-and-half, sage and pepper. On a lightly floured surface, roll out 1 pastry sheet to a 12-inch square; cut into 9 squares. Working with one square at a time, brush edges with egg and place 1 tbsp. filling in center. Fold together opposite corners of square, forming a triangle, and press together edges to seal; decorate with tines of a fork. Transfer to a baking sheet, brush with egg and prick top of each once with a fork. Repeat with remaining pastry sheet, filling and egg.
2. Bake 5 min. Reduce heat to 375 degrees F and bake until puffed and golden brown, about 15 minutes more. Serve warm with mustard (if using). Makes 18 turnovers.