For the pastry, mix the flour and salt together in a bowl then pour in the oil and egg yolk and mix together until the mixture resembles breadcrumbs. Gradually stir in enough warm water to form a smooth dough (you may not need all the water). Roll the dough into a ball, wrap in cling film and chill in the fridge for least half an hour
For the filling, boil the potatoes in a saucepan of salted water for 15-20 minutes, or until just tender, then drain. When cool enough to handle cut into 0.5cm/¼in cubes and set aside. Boil the carrots in a saucepan of salted water for 10-12 minutes, or until tender, then drain. Set aside.
Heat the two tablespoons of vegetable oil in a large frying pan over a medium heat then add the onion, turmeric, garam masala, mustard seeds, salt and black pepper and fry for about 8 minutes until the onions are softened and translucent and the spices aromatic.
Stir in the garlic, chilli, curry leaves, potato and carrot and cook for a further ten minutes then stir in the peas and cook for 2-3 minutes until heated through. Season with the lemon juice, stir in the coriander leaves and set aside to cool.
To make the samosas, cut the dough into six even-sized pieces and roll into balls. Use a rolling pin to roll each ball into a thin circle, about 15cm/6in diameter. Cut each circle in half to form two semicircles.
Take a semicircle and fold into a cone, brushing the edges with the reserved egg white to seal. Fill the cone about half-full with the samosa filling then brush the top edges with egg white and press tightly to seal. Repeat with the remaining dough and filling.
To cook the samosas half-fill a deep, heavy-based pan with vegetable oil and heat until a cube of bread dropped in sizzles and turns golden-brown in 30 seconds (CAUTION: hot oil can be dangerous. Do not leave unattended). Fry the samosas in small batches for 4-5 minutes, or until golden-brown and crisp. Remove from the oil with a slotted spoon and drain on kitchen paper. Serve.