Rinse the lentils under cold water until the water runs clear. Drain and set aside.
In a large pan, heat 2 tablespoons of olive oil over medium heat. Add the cumin seeds and let them sizzle for about 30 seconds.
Add the sliced onions to the pan, stirring frequently until they start to soften. Reduce the heat to low and continue cooking, stirring occasionally, until the onions are deeply caramelized and sweet, about 30-35 minutes.
Remove half of the caramelized onions and set aside for garnishing the dish later.
To the pan with the remaining onions, add the rinsed lentils, rice, ground cinnamon, salt, and pepper. Stir to combine with the onions.
Pour in the vegetable broth and bring the mixture to a boil. Once boiling, reduce the heat to low, cover, and simmer until the lentils and rice are tender and the liquid is absorbed, about 20 minutes.
Remove from heat and let it sit, covered, for 5 minutes. Then, fluff the lentil-rice mixture with a fork.
To serve, spoon the Mujaddara into bowls or onto plates, top with the reserved caramelized onions, and garnish with chopped parsley if desired.