Making the filling: Heat a large pan over high heat. Add the lamb, most of the chopped onions, the garlic, ginger, spices, chillies, chilli flakes, chilli powder and salt. Put a lid on and cook for about 15 minutes while stirring from time to time. The meat is ready when all the juices have been absorbed.
In the meantime, fill a large pan with water. Add a pinch of salt and bring to the boil. Put in the potatoes and boil over high heat for about 10 to 15 minutes, until they become soft. Transfer to a colander to drain.
Add the potatoes and peas to the lamb. Lower the heat to medium and cook for about 3 minutes. Turn off the heat and set aside to cool down. Mix in the rest of the onions and the coriander.
Making the pastry: In a bowl, combine together the flour, oil and salt. Dig a well in the middle then pour enough water to make a dough. Work with your fingers in the bowl until the dough becomes smooth. Divide into eight 5cm balls, cover them with a cloth and set aside.
Lightly flour your work surface then flatten out each ball into a thin 10cm diameter circle. Quarter each circle then stack up the quarters, dusting each layer with flour to prevent from sticking.
Making the paste: pour the flour into a bowl then add the water and stir well.
Brush a little paste along the arched edge of each pastry piece. Fold into a cone, sticking the curved sides together. Fill with a dessertspoonful of the meat and vegetables mixture. Brush some paste along the open sides of the pastry and seal. The samosas should have a triangular shape. Once all the pieces are done, leave them aside.
Line a dish with kitchen paper. Pour enough oil in a deep fryer or a heavy-bottom saucepan. Heat until the oil reaches 180°C. To check the temperature, carefully put in a cube of bread: it should brown within 60 seconds. Deep-fry the samosas (make several batches) for about 5 to 6 minutes, until they become golden brown. Using a slotted spoon, transfer the samosas onto the kitchen paper. Keep warm until you finish the last batch then serve.