Put a large Dutch oven or soup pot on the stove over medium heat.

Add the olive oil to the pot and heat it for about 30 seconds, until it looks loose and shiny.

Add the chopped onion, diced carrots, and diced celery to the pot. Sprinkle in 1 teaspoon of the salt and cook for 6 to 8 minutes, stirring often, until the vegetables soften and the onion looks translucent.

Add the sliced mushrooms to the pot. Cook for 8 to 10 minutes, stirring every minute or two, until the mushrooms give off their liquid and then start to brown around the edges.

Add the sliced garlic and chopped rosemary to the pot. Cook for 1 minute, stirring, until both smell fragrant.

Add the tomato paste to the pot. Stir it over the vegetables for 1 to 2 minutes, until it darkens slightly and coats the mushrooms and vegetables.

Add the rinsed lentils to the pot and stir for about 30 seconds so they are coated with the oil and tomato paste.

Pour the vegetable broth into the pot. Add the remaining 1/2 teaspoon salt and the black pepper, and stir well.

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

Lower the heat so the stew bubbles gently. Partly cover the pot with a lid and simmer for 20 minutes, stirring once or twice to make sure the lentils are not sticking to the bottom.

Add the drained chickpeas to the pot and stir them into the stew.

Keep the pot at a gentle simmer and cook for 10 to 15 minutes more, until the lentils are tender but still hold their shape and the broth has thickened into a stew.