Combine all dressing ingredients, except oil, in a large bowl. Slowly add oil, constantly beating with a whisk (or fork) until thoroughly blended. Adjust seasonings, to taste.
Drain and rinse lentils in a colander.
Chop bell pepper and celery (about 3/4 cup each), then place in the large bowl with dressing.
Thinly slice optional scallions, using both the green and white parts (to yield about 1/3 cup) and add to bowl. Coarsely chop parsley leaves and mince garlic, adding them to the bowl as you finish each one.
Stir in lentils, feta cheese, and capers.
Cover bowl and place in refrigerator to marinate at least 30 minutes before serving (for best flavor).