Place oven rack about 6 inches from top heating elements and preheat oven broiler.

Slice cauliflower into 1 inch florets. Peel onion, slice in half lengthwise, and then slice into 1/2 inch thick pieces. Toss cauliflower and onion with oil and salt. Spread out on rimmed baking sheet and broil until cauliflower and onion are tender and have some char, about 8 minutes, stirring once halfway through. Remove from oven and reserve about 1 1/2 cups roasted cauliflower florets for garnish.

In blender, place remaining cauliflower along with onion, broth, tahini, ginger, lime juice, garam masala, and cayenne and blend until smooth.

Serve soup garnished with reserved cauliflower florets, cilantro, and a drizzle of olive oil.