Set a large heavy pot or Dutch oven over medium-high heat for 1 minute.

Brown the chicken: Pat the chicken thighs dry with paper towels and season both sides with 1/2 teaspoon of the salt and all of the black pepper.

Add the olive oil to the hot pot.

Put the chicken thighs into the pot in one layer and cook for 4 to 5 minutes, until the bottom side is browned.

Turn the chicken and cook for 3 to 4 minutes more, until the second side has some brown color. The chicken does not need to be fully cooked yet.

Transfer the chicken to a plate.

Soften the vegetables: Add the onion, carrots, and celery to the same pot.

Cook the vegetables for 6 to 8 minutes over medium heat, stirring a few times, until the onion looks soft and the carrots start to lose their raw edge.

Add the garlic to the pot and stir for 30 seconds, just until fragrant.

Start the soup: Stir the lentils into the pot.

Pour the chicken broth into the pot and scrape the bottom with a wooden spoon to loosen any browned bits.

Bring the broth to a boil over high heat.