To prepare the filling: Heat the oil over medium heat and fry the onions till light brown. Add the mince, garlic and ginger. Stir-fry over low heat until almost dry. Add the turmeric powder, coriander powder, cumin powder, chilli powder and salt. Stir-fry until the mince is lightly browned. Add the water and the green peas and bring to the boil. Cover and simmer for 25 to 30 minutes. If there is any liquid left, remove the lid and cook the mince over medium heat until it is completely dry, stirring frequently. Stir in the desiccated coconut, garam masala powder, green chillies and coriander leaves.
Remove from heat and add the lemon juice. Cool thoroughly before filling the samosas.
To prepare the pastry: Add the butter and salt to the flour. Rub in well. Add enough warm water to mix a soft dough. Knead until the dough feels soft and velvety to the touch. Divide the dough into eight portions and roll into ten cm discs. Cut each into two. Roll each semicircle of the pastry to make an envelope. Moisten the straight edge with a little warm water. Fold the semicircle of the pastry in half to form a triangular cone. Join the straight edges by pressing them hard into each other. Make sure that there are no gaps.
Fill these cones with the prepared filling, leaving about 1/2 cm border on the top of the cone. Now moisten the top edges and press them hard together.
Deep fry the samosas in hot oil over gentle heat until they are golden brown and drain on absorbent paper.