Place beans in a large saucepan, and cover with salted water. Bring to a boil and then reduce heat to a simmer. Simmer over low heat for about 2 1/2 hours, covered. Add more water if needed. Add lentils, keeping it covered. Simmer until lentils and beans are tender and mixture is thick. Add cumin, lemon juice, and olive oil. Season with salt and pepper. Transfer to serving bowls, and garnish each with chopped green onion.