Over medium heat, melt the butter in a 2-quart saucepot. Add the rice and stir to coat well. Add the lentils, water and spices through the salt and pepper and bring to a boil. Cover the pot with a tight-fitting lid, reduce heat to a simmer and cook about 35 minutes. Check to be sure lentils are tender, then turn off heat and let steam, covered for 15-30 minutes. Meanwhile, heat the olive oil in a heavy skillet. Add the sliced onions and cook slowly until they are soft and golden. This will take 20-30 minutes; be sure not to let them burn. Turn the lentil-rice mixture onto a serving platter and spread the onions evenly over the top. Sprinkle the cinnamon evenly over all. Pass the yogurt to drizzle over and stir in as desired.