Bring water to a boil and add lentils, carrot, thyme, salt and pepper. Cover, reduce heat to low, and let simmer until lentils and carrots are tender, about 25 minutes.
Drain and transfer to a large bowl. Toss with red and white vinegars, olive oil, Dijon mustard, tomatoes, bell pepper and green onions. Serve warm, or to serve cold, chill cooked lentils and toss with cold ingredients before serving.