Place potatoes into a large pan and cover with water; bring to the boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and transfer potatoes to a bowl; coarsely mash.
Preheat oven 200 C / Gas 6.
Heat oil in a frying pan over medium-high heat; cook and stir onions, coriander seeds, curry powder, ginger, salt, turmeric, cumin, allspice, cayenne pepper and cinnamon until onion is lightly browned, about 5 minutes. Remove pan from heat and stir tomatoes and peas into onion mixture; pour into mashed potatoes and thoroughly mix. Cool completely.
Cut each sheet of pastry into 8 even triangles. Spoon filling onto the wide end of each triangle; fold corners over filling creating a triangular 'hat' shape. Pinch the dough together to form a seal. Brush egg white over each samosa and arrange on a baking tray.
Bake in the preheated oven until samosas are golden brown, about 15 minutes.