Preheat oven to 190°C. Cut chicken into very small cubes. Place it in a bowl, add the onion, coriander and cumin. Season with salt and pepper.
Cut the dough into 8 pieces. Put each dough piece cut side up and gently roll out into a circle about 5 inches in diameter, trying not to ruin the layers. Place the filling in the centre of each circle.
Dip your finger tips in flour. Bring two sides together, pinch tightly to seal and then pinch the third side to form a pyramid. Make sure all the edges are sealed well.
Place samoosas sealed side down on a baking sheet, previously covered with parchment paper. Brush samoosas with egg and sprinkle with sesame seeds. Bake for about 30 minutes, or until golden brown. Serve hot.