Fry the onion in oil until golden. Drain, reserving the oil for the rice. Set aside the onions for garnish.
In a large saucepan, cook the lentils in water for 30 to 40 minutes over medium heat. Make sure there is a little more than enough water to cover the lentils, about five cups. (Taste the lentils; they should be al dente.) After cooking, there should be only four cups of water left in the pan.
Rinse the rice and add to the lentils. Add six tablespoons of the oil from the fried onions. Add salt. Bring to a boil, and very gently stir the mixture with a wooden spoon. Simmer, covered, on low heat for another 30 minutes without lifting the lid.
After cooking, gently stir the mixture again, being careful not to crush the rice. Close the lid. After 10 minutes remove lid to allow the excess moisture to evaporate.
Serve with fried onions. Serving suggestion: Serve warm Mejadara with cold sour cream or yogurt on the side.