Set a large heavy pot or Dutch oven over medium heat so it is ready when the vegetables are chopped.

Rinse the red lentils in a fine-mesh strainer under cold water until the water runs mostly clear. Let them drain well.

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

Put the chopped onion and sliced celery into the pot. Add 1/2 teaspoon of the salt and cook for 6 to 8 minutes, stirring often, until the vegetables look soft and the onion is no longer raw.

Add the garlic, the 4 sliced sage leaves, and the black pepper to the pot. Cook for 30 seconds, stirring, until the garlic smells fragrant.

Add the rinsed red lentils to the pot. Stir for about 30 seconds so the lentils are coated in the oil and mixed with the vegetables.

Pour the vegetable broth into the pot. Stir and scrape the bottom of the pot so nothing is stuck there.

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

Lower the heat to keep the soup at a gentle simmer. Partly cover the pot and cook for 10 minutes, stirring once or twice, until the lentils start to soften.

Add the pumpkin puree to the pot. Stir until the soup is evenly orange and no streaks of pumpkin remain.

Add the drained chickpeas to the pot. Stir them into the soup.

Keep the pot at a gentle simmer and cook for 8 to 10 minutes more. The lentils should be very soft and the soup should look thick and smooth, with the chickpeas suspended throughout.