Add lentils to three quarts of water in a 4-quart pot; bring to a boil, reduce to simmer and let cook until tender – about 20 minutes. Drain well, tip into a large mixing bowl, drizzle with the olive oil and red wine vinegar, and sprinkle with the salt and peppers. Stir well. Add the onion, celery, fennel, carrot, parsley, and olives to the lentils; stir again. Pour the vinaigrette over the salad and toss well. Carefully stir in the salmon. Taste and adjust seasonings. Using a large ice cream scoop or ladle, place about 1 ¼ cups of salad at the center of the tomatoes on each plate for 4 main servings. Use about ¾ cup each for 6 first-course servings or a bit more than ½ cup for each of 8. Add a few of the reserved kalamata olives around the edges of the plate. Garnish the plates with a thyme sprig or mint and a wedge of lemon, if using.