Combine 4 cups water and 1/2 teaspoon salt in heavy large saucepan over high heat. Add lentils; bring to boil. Cover, simmer until lentils are tender, 20 minutes. Drain well; rinse with cold water and drain again. Place in a large bowl.
Whisk 1/2 cup olive oil, vinegar, 1 Tablespoon cumin, 1 tablespoon chili powder, 1 clove garlic and 1/2 teaspoon salt. Pour 1/2 cup of dressing over lentils and toss.
Heat 2 Tablespoons olive oil in large frying pan over high heat. Add chopped onion; saute until brown and soft, 5 minutes. Add chicken; saute 2 more minutes. Add 1 teaspoon salt, 1 Tablespoon cumin, 2 teaspoons chili powder and 1/4 teaspoon cinnamon. Saute until chicken is cooked through, 3 minutes longer.
put chicken on lentils, drizzle with remaining dressing.