Preheat oven to 450°. Put cauliflower florets in a large bowl and add 1 1/2 tbsp. oil, 1 tsp. paprika, 1/2 tsp. salt, the cumin, chile flakes, and garlic. Toss to coat thoroughly.
Spread florets evenly on a large rimmed baking sheet and roast, stirring once, until florets are cooked through and a little crispy, 18 to 20 minutes. Let cool.
Put 1/2 cup water in a blender with roasted cauliflower and garlic, lemon juice, remaining 3 tbsp. oil, the tahini, and remaining 1 1/2 tsp. salt. Blend, adding more water if needed (up to 1/4 cup) and scraping sides often, until you have a creamy purée, about 4 minutes. Taste and add more salt or lemon juice if you like.
Spoon into a serving bowl and sprinkle with a pinch of smoked paprika.
Make ahead: Up to 3 days, chilled airtight.