Set the oven to 450 degrees Fahrenheit (230 degrees C).
Grease two or three baking sheets with parchment paper liberally.
Combine the flour, cornstarch, garlic, baking powder, ginger, and salt in a large mixing bowl.
Make a well in the center, add the sparkling water, and whisk the mixture with a fork to create a smooth batter.
To equally coat the cauliflower florets in the batter, add them and gently toss them.
Place the florets on the baking sheets, leaving enough space between each one to crisp up in the oven.
Bake for about 10 minutes, turning once, until crispy and golden brown.
For Sauce:
Place the ketchup, brown sugar, soy sauce, vinegar, sriracha hot sauce, sesame oil, and garlic in a small saucepan.
Mix thoroughly with a wooden spoon, then simmer gently with bubbling.
Cook, stirring frequently, for about 10 minutes, or until it reduces to a glossy consistency that coats the back of a wooden spoon.
To Assemble:
Take the cauliflower out of the oven and place it in a bowl.
Drizzle with the sweet and sour sauce, then toss in the majority of the cilantro and scallions, leaving some for garnish, and gently toss to coat each floret.
Before serving, transfer to a serving plate and top with the remaining cilantro and scallions.