Set a Dutch oven or other medium soup pot on the stove over medium heat.

Add the olive oil to the pot and let it warm for about 30 seconds.

Add the onion, carrots, and celery to the pot.

Cook the vegetables for 6 to 8 minutes, stirring every minute or two, until the onion looks soft and the carrots start to lose their raw edge.

Add the garlic to the pot.

Stir in the tomato paste and cook for 1 minute, stirring often, until it looks a shade darker.

Add the lentils and vegetable broth to the pot and stir once more.

Turn the heat to high and bring the stew to a full boil.

Lower the heat to low and cover the pot partway with a lid so steam can escape.

Simmer for 25 minutes, stirring once or twice, until the lentils have started to soften.