Brown the sausage: Set a large Dutch oven or heavy soup pot over medium heat and let it warm for about 1 minute.

Add the olive oil to the pot. Add the Italian sausage and cook for 5 to 6 minutes, breaking it into small pieces with a wooden spoon, until it is browned in spots and no longer pink.

Use a slotted spoon to transfer the sausage to a plate, leaving the fat in the pot.

Soften the vegetables: Add the onion, carrots, and celery to the pot. Add about 1/2 teaspoon of the kosher salt and cook over medium heat for 6 to 8 minutes, stirring now and then, until the vegetables soften and the onion looks glossy.

Add the garlic, dried rosemary, and red pepper flakes to the pot. Stir for 30 seconds, just until the garlic smells fragrant.

Add the tomato paste to the pot. Stir it into the vegetables for 1 minute, until it darkens slightly and coats the bottom of the pot.

Start the stew: Add the lentils to the pot and stir so they are coated with the tomato and vegetable mixture.

Pour the chicken broth and the 2 cups of water into the pot. Scrape the bottom of the pot with the spoon to loosen any browned bits.

Return the browned sausage and any juices from the plate to the pot.

Raise the heat to high and bring the liquid to a boil.

Lower the heat to low so the stew bubbles gently. Cover the pot partway and cook for 20 minutes, stirring once or twice.

Add the drained chickpeas to the pot. Keep simmering, partly covered, for 10 to 15 minutes more, until the lentils are tender but still holding their shape.