Preheat oven to 425 F. Prepare the batter by mixing flour, cornstarch, chili powders, salt, pepper, and water in a bowl. Whisk until the batter is smooth and runny, but still thick enough to adhere to the cauliflower. Set aside a plate with panko breadcrumbs.
Dip each cauliflower in this batter and then the panko breadcrumbs. Place on a lined baking sheet. Bake for 30-40 minutes or until crisp. The timing will depend on your oven and the size of the florets.
For the manchurian sauce, heat oil in a medium pan or wok over medium heat.
Add garlic, green chilis, and ginger. Lightly fry for a couple minutes. Add onions and cook until translucent. Add bell peppers and carrots. Cook for 4-5 minutes, allowing them to soften and mostly cook through.
Add chili garlic sauce or sriracha, and ketchup. Combine with the vegetables. Add red chili powder and kashmiri chili powder. Finish with soy sauce and cornstarch slurry. Stir and reduce until it is at your desired thickness.
Toss in the baked cauliflower, garnish with scallions, and enjoy immediately with rice or noodles!