Heat the oven to 220 C / 425 F with a rack in the middle. Toss the carrot chunks with the olive oil and 1/2 teaspoon salt on a 33 x 46 cm / 13 x 18 inch rimmed sheet pan, spread them in a single layer, and roast for 15 minutes.
Flip the carrots with a spatula and tuck the unpeeled garlic cloves among them. Roast for 15 to 20 minutes more, until the edges are dark brown and a fork squashes a chunk with no resistance.
While the carrots finish, toast the cumin seeds in a dry 20 cm / 8-inch skillet over medium heat for 1 to 2 minutes, until fragrant and a shade darker. Grind them in a mortar or spice grinder, then drain and rinse the chickpeas.
Let the carrots and garlic cool on the pan for 10 minutes, then squeeze the soft garlic out of its skins.
In an 11-cup / 2.6 L food processor, blend the tahini and lemon juice for 1 minute, until thick and pale. Add the harissa, garlic, cumin, and remaining 1/2 teaspoon salt and process for 30 seconds.
Add the carrots and chickpeas and process for 2 minutes, scraping down the bowl once. With the motor running, add the ice water 1 tablespoon at a time until the hummus is smooth, glossy, and spoonable, then taste and adjust the salt and lemon.
Spread the hummus in a shallow bowl, swirl the top with the back of a spoon, and drizzle with the extra-virgin olive oil. Scatter the pepitas and parsley over the top and serve at room temperature.