Drain the cooked lentils and transfer to a bowl. Cut the roasted pepper into strips and the cucumber into chunks.
Add the pepper and cucumber to the lentils. Tear the mint leaves into small pieces and toss through.
Make a dressing with olive oil, red wine vinegar, salt and pepper. Pour over the salad and toss well.
Finally, crumble the feta over the salad. Let the salad rest briefly so the lentils absorb the dressing, then serve at room temperature.