Place the squash, cauliflower, shallots and chickpeas on a half-sheet pan. Drizzle with 2 tablespoons of the olive oil and sprinkle with the zaatar and salt, then toss to coat well. Roast, turning once, until the veggies are tender and just starting to brown, 20 to 25 minutes.
While the veggies cook, make the dressing. Whisk together the tahini, maple syrup, lemon juice, a pinch of salt and the remaining tablespoon of olive oil. While whisking, slowly drizzle in 1 tablespoon of the hot water at a time, until the mixture is smooth. Season with salt.
Remove the roasted veggies from the oven. Top with the arugula, pomegranate seeds, pistachios and goat cheese, then drizzle with the tahini dressing.