In a medium bowl, cover the rice with cold water. Set aside.

Line a plate with two paper towels. Heat the olive oil in a large pot over high heat. Add the onions and cook, stirring constantly, until the onions are deeply browned and start to crisp around the edges, about 20 minutes. Use a slotted spoon to transfer to the plate and set aside. They will continue to crisp as they cool. Reserve the oil from cooking the onions.

Add the lentils to another pot, add water and stir to combine. Bring everything to a boil, then reduce the heat to medium-low and cover with a tight-fitting lid. Cook for 15 minutes.

Drain the soaked rice, rinse and transfer it to the pot with the lentils along with the salt. Stir once, then cover again with a tight-fitting lid and cook undisturbed for 15 minutes.

Remove the pot from the heat. Rest, covered, for 5 minutes longer. Add the cumin and fluff the rice and lentils with a fork to combine.

Pour the reserved oil on top of the mujadara. Then spoon the crispy onions on top. Serve warm, with cucumber yogurt sauce if desired.