For the filling, combine butter, diced peaches, 3 tablespoons sugar, and salt in a medium saucepan. Cook over medium high heat, stirring frequently, until butter is melted and juices are bubbling. Reduce heat and simmer for 15 to 20 minutes, stirring frequently, until fruit has softened. Whisk together cornstarch in water and add to saucepan. Simmer for 1 to 2 minutes, until mixture has thickened. Transfer to a bowl and chill until ready to use.
Line two baking sheets with parchment paper. Cut the pie pastry into 4 pieces (pie shaped). Repeat with second disc of dough and preheat oven to 400°F.
Working with 1 dough at a time, place two teaspoons of peach filling in the center of the triangle. Top with 1 or 2 peach slices and sprinkle with sugar. Brush edges of pastry lightly with water and fold over at a diagonal to form a triangle. Seal the edges of each turnover by crimping with a fork and vent tops with a sharp knife. Chill assembled turnovers in the refrigerator for 10 minutes.
Just before baking, brush turnovers with cream and sprinkle with sugar. Bake for 26 to 30 minutes, until golden brown. Cool turnovers on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
