Heat the oven to 425°F and set a rack in the middle. Place a large pot or Dutch oven on the stove so it is ready for the stew.

Put the cauliflower florets on a sheet pan. Drizzle them with 2 tablespoons of the olive oil and sprinkle with 1/2 teaspoon of the salt.

Toss the cauliflower on the sheet pan until it is evenly coated. Spread it into one layer so the pieces can brown instead of steam.

Roast the cauliflower for 25 to 30 minutes, turning it once halfway through, until the edges are dark golden and the stems are just tender.

While the cauliflower roasts, pour the remaining 1 tablespoon olive oil into the large pot and set it over medium heat.

Add the chopped onion to the pot with 1/2 teaspoon of the salt. Cook for 6 to 8 minutes, stirring often, until the onion is soft and lightly golden.

Add the sliced garlic and the ground cumin to the pot. Stir for about 1 minute, until the garlic smells fragrant and the cumin darkens slightly.

Add the rinsed lentils to the pot. Stir them for about 30 seconds so they are coated with the oil and onion mixture.

Pour the vegetable broth into the pot and raise the heat to high. Bring the broth to a boil.

Lower the heat so the liquid stays at a gentle simmer and cover the pot partway with a lid. Cook for 18 to 22 minutes, stirring once or twice, until the lentils are tender but still hold their shape.

Add the drained chickpeas to the pot. Simmer for 5 minutes so the chickpeas heat through and the flavors settle together.

Put the tahini and the lemon juice in a small bowl. Ladle in 3 tablespoons of hot broth from the pot and whisk until the mixture is smooth and pourable.