Heat the olive oil in a wide skillet over medium-high heat. Add the sliced onions and cook, stirring frequently, until deeply golden and crisp. Remove the onions and reserve the oil.
In a large, heavy-based saucepan, combine the lentils, rice, water, salt, and cumin (if using). Bring to a boil over medium-high heat.
Reduce the heat to medium-low and cook uncovered for about 20 minutes, until the lentils are al dente.
Stir a few tablespoons of the reserved onion oil into the pot, cover, and simmer over low heat for about 10 minutes, until the lentils and rice are tender.
Serve warm or at room temperature, topped with the fried onions.