First make the filling. Roughly crush the coriander and cumin seeds using a pestle and mortar. Heat 2 tablespoons of oil in a frying pan set over a medium heat and add the crushed seeds, frying them until they smell aromatic. Add the onion, garlic, ginger and chilli to the pan and fry for 3–4 minutes until the onions are translucent. Add the rest of the spices and the salt to the pan and stir through for 1 minute. Place this mixture into a large mixing bowl and add the potato, peas and coriander. Combine well. To prepare the samosas, make a flour paste by adding 4 tablespoons of water to the flour and stir until you have a thick, glue-like mixture. Take a pastry sheet and cut it in half diagonally so that you have two triangles. Take one triangle, brush the edges with the flour paste and fold it in half so that you are left with a smaller triangle. Place a finger in the middle of the bottom edge of the triangle while folding the right side over until it hits your finger, then glue down the edge. Bring the left side over the right until you have a pointed cone shape, then glue down the edge. Open up the cone in your hand and fill with a heaped teaspoon of the samosa filling. Moisten the excess pastry around the edges of the cone and fold one edge inside the cone so that it covers the filling. Bring the last edge over the outside of the samosa so that you are left with a tidy triangle. Repeat with the rest of the pastry until you have 15 samosas. When you are ready to fry, pour the oil into a pan to a depth of 5cm and set over a medium-high heat. Test whether the oil is hot enough by adding a small amount of pastry sheet; if it sizzles and rises to the surface, the oil is ready. Add the samosas to the pan in small batches, as overcrowding can cool the temperature of the oil. Fry the samosas until they are golden brown. This should take 4–5 minutes. Drain the samosas on kitchen paper to get rid of the excess oil, then serve immediately while still crisp and hot. Storage notes: These can be made in advance and fried a second time just before serving. Alternatively, they can be frozen on the day of cooking for up to 3 months.