Place a pizza stone on the lowest oven rack and another rack one level below the top; preheat oven to 450°F (232°C). Line two baking sheets with parchment paper.
Make the roasted onion cream: toss the chopped onion and halved garlic with 1 teaspoon oil, salt, and pepper on one prepared baking sheet. Roast on the top rack until edges char slightly, about 15 minutes. Let cool a few minutes.
Blend the roasted onion and garlic with soaked cashews, 1 tablespoon oil, nutritional yeast, lemon juice, 1/4 cup water, and salt and pepper until completely smooth. Transfer to a small bowl and set aside.
Whisk the chickpea flour, 3/4 cup water, salt, and pepper in a large bowl until smooth and the consistency of thin pancake batter.
Toss the small cauliflower florets in the batter until evenly coated. Using a slotted spoon, transfer florets to the second prepared baking sheet, tapping off excess batter and spacing them in a single layer.
Bake the battered cauliflower at 450°F for 20 minutes, until the batter is dry and slightly browned; let cool briefly.
In the large bowl (rinsed), whisk together the Buffalo hot sauce and 1 tablespoon oil. Add the baked cauliflower and toss to coat, breaking up any large pieces. Discard loose baked batter bits if desired.
Return the sauced cauliflower to the baking sheet and bake for an additional 15 minutes, until edges brown slightly and the exterior is dry. Remove and let cool while you prepare the pizza.
Increase oven temperature to 500°F (260°C). On a floured surface or on parchment, stretch/roll the pizza dough into a 12-inch circle and place it on a floured peel or on parchment (trim parchment to leave a 1/2-inch border).
Spread the roasted onion cream evenly over the dough, leaving a 1/2-inch border. Arrange all the Buffalo cauliflower on top and gently press the florets into the sauce and dough.
Slide the pizza onto the hot stone (or transfer with the parchment) and bake for 7 minutes. Rotate the pizza, then bake another 5 minutes, until the crust browns in spots and the onion cream has set near the crust.
Remove the pizza, slice, and top with baby arugula and sliced green onions. Drizzle with vegan ranch and serve immediately.