Mise en place - Dice the ham into ½-inch (or smaller) cubes. Peel, and finely dice the red onion. Wash, trim and chop the celery. Mince the parsley, enough to yield ½ cup.
Cook the lentils and toss with the ham - Sauté the diced onion until very tender. Add 2 t thyme and the lentils, and stir to coat. Add 6 cups water and bring to boil. Reduce heat to medium-low. Cover and simmer until lentils are very tender, about 30 minutes. Drain lentils after checking for doneness. Toss the diced ham with the drained lentils.
Make the dressing - Whisk ½ cup oil, ¼ cup red wine vinegar, and 3 T Dijon mustard in a small bowl to emulsify.
Dress the lentils and serve - Pour the dressing over the lentils and ham. Fold in the red onion, celery, and parsley. Season generously with salt and pepper and serve at room temperature.