Preheat the oven to 350°F. Heat the olive oil in a sauté pan over medium heat. Add the onions and sauté until golden, 15 to 20 minutes. Combine ½ cup of the sautéed onions and the carrots in a small bowl and set aside.

Place the rice and lentils in a strainer, rinse under cold running water, and drain. Season with salt and pepper.

Spread the carrot and onion mixture evenly over the bottom of a large casserole dish with a tight-fitting lid (or use 2 layers of aluminum foil to cover the dish). Spread the rice mixture evenly over them and top with the remaining onions. Pour the boiling water or stock over the rice and cover the dish tightly with the lid or foil. Bake for 1 hour or until all the water is absorbed and the lentils are firm yet break when pressed between a spoon and the side of the dish. Serve warm.