Heat oil in a deep sauce pot over medium heat and sauté onions until translucent and caramelized, 20-25 minutes, stirring frequently. Sprinkle with salt and pepper to taste. Remove from pan and set aside.
In the same pan, add lentils and increase heat to medium-high. Toast lentils for 60 seconds then add 6 cups water. Bring pot to a boil then reduce heat to low and simmer until lentils are halfway cooked, about 15 minutes.
Add rice, salt and pepper to the pot and bring mixture to a boil. Stir once, cover with lid, then reduce heat to low. Cook until all liquid is absorbed, about 15 minutes.
Fluff lentils and rice with a fork before serving with caramelized onions. Serve with plain yogurt or Lebanese Cucumber Laban if you wish.