Dice 3 of the onions. Heat a large heavy skillet and add 2 tablespoons of the olive oil and the diced onions. Saute until quite brown and set aside. Put the lentils and water in a large covered pot. Bring to a boil, covered, then simmer for about 15 minutes. Add the cooked onion to the lentils along with the rice and salt. Cover and simmer for 20 minutes, until rice and lentils are soft.
Remove from heat and let stand 5 minutes. Slice the remaining 2 onions into rings. Heat the skillet again and saute the rings in the remaining oil. To serve, top the lentils with the sauteed onion rings.