Place potatoes, cream, mustards and garlic in a small saucepan. Simmer over a medium heat for 20 or 30 minutes, or until the potatoes are soft. Season and set aside to cool slightly. Stir through the camembert and spring onions.
Pre-heat oven to 180C. Line 2 baking trays with baking paper. Cut a 20cm circle from each of the pastry sheets, and top half of each of the circles with a 1/4 of the potato mixture, leaving a border.
Brush egg around the edge of the pastry circles and carefully fold in half to enclose the filling. Crimp the edges with a fork to seal.
Brush the tops of the pasties with a little egg, and sprinkle with sesame seeds. Bake for 15 to 20 minutes, or until golden brown.