For the cauliflower, preheat the oven to 400F. Line a large baking sheet with foil and drizzle on and spread around 1 tablespoon of vegetable oil (or spray the sheet with cooking spray).
In a large bowl, whisk together the flour, cornstarch, baking powder, salt, black pepper, garam masala spice mix, hot chili powder, garlic, ginger, soy sauce, and water (adding enough water so the batter comes together, but isn’t too thin).
Add the cauliflower to the batter and toss around to coat each piece.
Pick up a piece of cauliflower, drain off the excess batter, and place it on the prepared baking sheet; continue lining up the cauliflower this way until finished.
Bake until the cauliflower is tender inside and golden outside, about 30 minutes, rotating the tray once.
Meanwhile, make the sauce. To do so, in a medium bowl, whisk together the soy sauce, tomato paste, coconut palm sugar, apple cider vinegar, hot sauce, crushed red pepper flakes, turmeric, garam masala spice mix, hot chili powder, black pepper, and water. Set the spicy sauce mix aside for now.
Heat the oil over medium heat; once hot, add the onion and cook until softened, but not yet browned, about 3 to 4 minutes, stirring occasionally. Add the garlic and ginger and cook 1 minute more, stirring constantly.
Add the spicy sauce mix and bring to a boil, then whisk in the cornstarch slurry and bring to a simmer. Remove from heat, then carefully taste and add salt as desired.
In a large bowl, toss the hot cauliflower with the sauce to coat.
Transfer to a serving dish and top with the scallion; serve.