Rinse the lentils in a fine-mesh strainer and see if there are any particles to remove. Peel and chop the onion and garlic. Heat olive oil on medium-high in a large pot and add the onion. After a couple of minutes, stir in the garlic. Cook until the onions are soft and almost starting to brown, lowering the heat a bit if needed.
While the onions and garlic are cooking, rinse and peel the carrots, chop off and discard the ends, and slice the carrots into ½-inch-thick coins. Wash the celery, trim off and discard the ends, and dice the celery (cut into small pieces).
When the onions are almost starting to brown, add the celery, carrots, and tomato paste to the pot and stir to combine. Cook for 10 minutes, adding a little of the chicken stock if the oil evaporates. Then add all of the chicken stock, salt, pepper, red pepper flakes, Liquid Smoke, cumin, thyme and lentils. Raise the heat and cover the pot until it boils.
When the pot is boiling, lower the heat and simmer uncovered for 30-45 minutes for lentil soup or up to one hour and 15 minutes for lentil stew. For soup, taste the lentils after 30 minutes to see if they're soft enough for your liking.
Serve with freshly grated Parmesan on top. Store leftovers in the refrigerator for up to four days. This freezes well.
Notes
Liquid Smoke provides great flavor without using bacon. It should be located near the other condiments in the grocery store.
The nutritional information listed is for making this with chicken stock. If using vegetable stock, each serving will have 395 calories, 61 grams of carbs and 21 grams of protein.
I store unused tomato paste in my freezer by placing a 2-3 tablespoons of it in plastic wrap and then putting those little packets in a freezer bag so they're ready for recipes like this one.