Put the lentils in a pot together with 3 cups of water, cover, and turn on the heat to medium. When the water comes to a boil, turn down the heat to low and cook at a gentle simmer for 15 minutes. The lentils are done when they are tender but firm and with their skins intact.
Drain the lentils and put them in a serving bowl. Dress them immediately with olive oil, salt, and a liberal grinding of pepper.
When the lentils are lukewarm, add the cucumber, the vinegar, the onion, toss well, and serve.