Slice 1 head cauliflower into 1-inch florets, and cut 1 yellow onion into 1-inch-thick pieces. Toss with 4 whole peeled garlic cloves, 1 tablespoon oil and ½ teaspoon salt. Spread on a rimmed baking sheet and roast 25 minutes at 400 F, stirring halfway through. Blend together 4 cups vegetable broth, roasted cauliflower/garlic/onions, ¼ cup tahini, juice of ½ lemon and 2 teaspoons za’atar seasoning until smooth.