Stir-fry the onion in lard until golden brown. Removed from the heat, sprinkle with paprika. Stir in the pieces of chicken, the green pepper and tomato, salt, cover, and cook gently in its own juice. When the liquid is reduced, brown lightly both sides of the meat, then remove from the pot. Stir the flour in the lard, brown slightly. Pour in 3/4 cup of cold water and bring to boil. Replace the meat in the pot. Stir in the sour cream just before serving.