Set a rack about 6 inches below the broiler. Put a large Dutch oven or soup pot on the stove over medium heat.

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

Put the sliced leeks, diced fennel, and diced carrots into the pot. Add a good pinch of salt and cook, stirring often, for 8 to 10 minutes, until the vegetables are soft and glossy.

Sprinkle the dried thyme into the pot. Stir for 30 seconds, just until it smells fragrant.

Add the rinsed green lentils to the pot. Stir for 1 minute so the lentils are coated with the oil and vegetables.

Pour the vegetable broth into the pot and add 2 cups of water. Raise the heat to high and bring the soup to a full boil.

When the soup boils, lower the heat to medium-low so it bubbles gently. Partly cover the pot and cook for 25 minutes.

Check the lentils after 25 minutes. They should be swollen and mostly tender, with just a little firmness in the center.

Add the drained chickpeas to the pot. Stir them into the soup and keep cooking at a gentle simmer for 8 to 10 minutes, until the lentils are fully tender and the broth has a little body.

Taste the soup and add more salt and a few grinds of black pepper as needed. If the soup looks thicker than you want, stir in a splash of water.

Put the bread slices on a sheet pan or other broiler-safe pan in a single layer.

Divide the grated Gruyère over the bread slices, spreading it all the way to the edges so it melts evenly.