In a large stockpot or Dutch oven set over medium heat, add the olive oil. Once hot, add the onion, celery, and carrots; sauté for about 8 minutes. Add the garlic and sauté until fragrant, about 2 minutes.
Sprinkle in salt, pepper, cumin, and red pepper flakes. Add the tomato paste, vinegar, and lentils. Stir frequently, cooking for 5 minutes.
Add bay leaf and pour in broth. Bring the mixture to a boil. Cover and simmer for 25-30 minutes or until the lentils are fully cooked.
Stir in chopped parsley (optional) and serve warm.
Notes
-If adding sausage, cook it in the pan first, remove it from the pan, and set it on a plate. DO NOT ADD THE OLIVE OIL. Sauté the vegetables in the oil from the sausage, then return the sausage to the pot after adding the lentils.-If adding chicken, add the pulled rotisserie chicken (about 2 cups) during the last 10 minutes of cooking time.To STORE the Lentil soup, let it cool, then transfer it to an airtight container. It will keep well in the fridge for up to 3-5 days. To FREEZE, place in a freezer-safe container and it will stay good for up to 4-5 months.To REHEAT, thaw in the fridge overnight if frozen, and warm it on the stove over medium heat until hot. Add a splash of broth if needed to maintain the desired consistency.