Place the lentils in a heat-proof bowl and cover with boiling water. Set aside for 15 minutes, then drain. (Note: Don’t omit this step. Though lentils don’t technically have to be soaked, skipping the step in this recipe means they will take a little longer to cook AND the amount of broth when cooking will not be enough.)
Meanwhile, heat the oil in a large, lidded sauté pan or Dutch oven. When hot, add the onions, leeks, salt, and pepper and sauté over medium heat for about 8 minutes, or until the onions are lightly golden in spots. Add the garlic and thyme and cook for 30 seconds or until the garlic is fragrant. Add the tomato paste and cook, stirring, for another 30-60 seconds. Add the drained lentils, celery, carrots, and chicken broth. Bring the mixture to a boil, and then cover and reduce the heat to maintain a gentle simmer (low to medium low heat) for 20 minutes, or until the lentils are tender but not mushy. (Check a few minutes early and add time if necessary. Older lentils may take longer to soften. If this is the case and moisture runs low, add another splash of broth or water as needed.)
Stir in the vinegar—make sure the measuring spoon is level or just slightly scant—you can always add more. (Did you know? The “meniscus,” or little bulge of liquid that forms when a cup is overfilled but not quite overflowing, can result in more liquid, and in this case more tang, than you may want.) Taste and add an extra sprinkle of salt, pepper, or vinegar, as desired.
Serve as a side dish, as a base for the salmon recipe linked in the recipe notes, or as a hearty meatless stew.