Heat the oil in a saucepan over a medium heat and add the cardamom. Fry for 10 seconds, then add the onions. Fry for 7–8 minutes more, until softened and changed in colour slightly.
Mash or grate the garlic and ginger add to the pan and fry for 1 minute, stirring frequently. Stir in the tomato purée and fry for another 2 minutes. Add the lamb and fry for another minute, stirring continuously.
Add the chilli powder, cook, stirring, for 1 minute, until the mince is evenly coloured all the way through. Add the dried fenugreek leaves, season to taste with salt and cover with a lid. Simmer on a low heat for 4 minutes until mixed well with the keema. Turn off the heat, add the garam masala, coriander, green chilli and lemon juice. Tilt the saucepan slightly and drain out any excess oil with a spoon. Set aside to cool completely.
Mix the flour with 2 tablespoons water in a small bowl to make a paste – this will be used to seal the samosas.
Unroll a single sheet of pastry over a flat surface. Return the remaining pastry to the packet to stop the sheets drying out. With the shortest side of the pastry sheet facing you, cut the pastry into two long strips, each 10cm/4in wide.
Scoop a heaped teaspoon of the keema onto the right-hand corner of the filo and mould into a triangle shape. Take the right corner and fold diagonally to the left, enclosing the filling and forming a triangle. Fold again along the upper crease of the triangle. Keep folding in this way until you reach the end of the strip. Seal the end of the samosa with the flour paste and repeat with the remaining filling and pastry.
Shallow fry your samosa over a high heat until they are browned. Remove the excess oil with a paper towel and serve.