Mix the flour and salt in a bowl. Add the water and oil, knead into a soft, smooth dough, cover and leave to rest at room temperature for approx. 30 mins.
Heat the oil in a pan. Sauté the onion and all the other ingredients up to and including the turmeric. Add the potatoes and cook briefly. Add the water, lemon juice and salt, cover and simmer gently for approx. 10 mins. Mash the potatoes with a fork, mix in the peas and coriander.
Divide the dough into 6 portions, shape into balls, flatten a little. On a lightly floured surface, roll the balls into circles (each approx. 17 cm in diameter). Cut the circles in half, brush the cut edges with a little water. Fold each half into a cone shape, press the edges together firmly. Spoon in the filling to approx. 1 cm below the rim, brush the edges of the dough with water, press together firmly. Shape and fill the remaining samosas in the same way, place on a tray lined with baking paper.
Approx. 20 mins. in the centre of an oven preheated to 220°C.