In a large bowl whisk together oil, vinegar, thyme, tomatoes, and salt and pepper to taste. Add lentils to a large saucepan of salted water and bring water to a boil. Cook lentils at a bare simmer until just tender, about 20 minutes. Drain lentils in a sieve and rinse gently. Toss lentils well with dressing and gently stir in Feta.