Soak the lentils in cold water for 12 hours. Fry the sliced onion in the butter till brown, add the curry powder, mix well and add the lentils. Cook these for 10 minutes, stirring frequently. Then put 1gill of hot water in the saucepan, season with salt, and simmer till the lentils are quite tender and have absorbed all the water. Ten minutes before serving, add the halved hard-boiled eggs.