Arrange two racks in your oven and preheat it to 425ºF/218ºC (use the convection setting, if your oven has one). Line two baking sheets with parchment paper.
Make the cauliflower batter: mix together the all-purpose flour or rice flour, cornstarch, garlic, ginger, chili powder, and salt in a large bowl. Gradually pour in the water and whisk until you have a lump-free and smooth batter that is quite thick and viscous (it should feel a little heavy with some resistance).*
Add a few florets to the batter and use a silicone spatula to toss the florets all over, coating all the crevices. Continue adding a few more florets until they're all coated.
Spread the panko bread crumbs out on a large plate or a shallow bowl. Using one hand, dip a few florets into the panko (keep the other hand clean for ease). Transfer the florets the sheet pans (divide the florets evenly between the two pans, make sure they aren't touching). Repeat with rest of florets.
Bake for 20 minutes. Take the pans out of the oven, rotate them by 180º and switch their positions in the oven for even baking. If already quite brown, reduce the temperature to 400ºF. Bake for another 15 to 20 minutes, until cauliflower is crispy and browned in spots.
While cauliflower is baking, make the Manchurian Sauce. Mix together soy sauce, chili sauce, ketchup, sugar, sesame oil, and vinegar in a bowl. Set aside. In a small bowl, stir together the cornstarch and water until well combined.
Heat a flat-bottomed wok** over medium-high heat until it just starts to smoke. Then add 3 tablespoons of the oil and swirl the pan to get it up the lower sides of the pan. Add the ginger, garlic, and chili peppers, Cook for 1 minute, shaking the pan back and forth or stirring frequently. Add the shallots, scallion whites and light green parts, and green bell pepper, and season with a pinch of salt. Cook for 3 to 4 minutes, stirring or shaking the pan often, or until the aromatics start to get some color.
Add the sauce and stir fry for 1 minute, stirring frequently. Stir the cornstarch slurry again to incorporate, and then pour it into the pan. Mix well and stir until the sauce turns glossy and thickens a bit (should be very quick). Turn off the heat and allow the sauce to come to room temperature.
Coat the cauliflower in sauce. When the florets are done frying, add them to the sauce (off the heat) and mix gently until well coated. Garnish with the reserved scallion greens and chopped cilantro. Serve immediately.