Rinse lentils under cold running water. In 4-quart saucepan, combine lentils and 8 cups of water; heat to boiling over high heat. Stir in carrots and apples, heat to boiling. Reduce heat to low and simmer until lentils are tender, 15 to 20 minutes, drain.
Meanwhile, from lemons, grate 2 tsp. peel and squeeze 1/4 c. juice. In a small bowl, with wire whisk or fork, mix oil, vinegar, sugar, salt, pepper, lemon juice, and lemon peel.
To serve, in a large bowl, toss spinach with 1/3 c. dressing. Stir remaining dressing into lentil mixture. Place spinach leaves on salad plates and top with lentils. Sprinkle with parsley.