Mix flour and salt, then stir in oil and egg yolk until crumbly.
Add warm water to form a smooth dough, wrap in cling film, and chill for 30 minutes.
Boil potatoes for 15 minutes, drain, and cut into 0.5cm cubes.
Boil carrots for 10 minutes until tender and drain.
Fry onion with turmeric, garam masala, mustard seeds, and pepper for 8 minutes.
Add garlic, chilli, curry leaves, potato, and carrot, and cook for 10 minutes.
Stir in peas, cook until heated, then add lemon juice and coriander.
Roll dough pieces into 15cm circles and cut each in half.
Fold each semicircle into a cone, seal with egg white, and fill with the vegetable mixture.
Deep-fry the samosas in small batches for 4 minutes until golden-brown and crisp.