Boil the cauliflower florets in salted water for 2-3 minutes. Drain completely and pat dry.
Whisk the all-purpose flour, 1/4 cup cornstarch, ginger-garlic paste, red chili powder, and salt with enough water to form a smooth, thick batter.
Heat the deep frying oil in a wok or heavy-bottomed pan. Dip the dried cauliflower florets into the batter to coat evenly, then carefully drop them into the hot oil.
Fry the florets until golden brown and crispy. Remove using a slotted spoon and drain on paper towels.
In a separate wok, heat 2 tablespoons of oil over high heat. Add the finely chopped garlic, ginger, green chilies, and the white parts of the spring onions. Stir-fry for 1 minute.
Add the diced capsicum and stir-fry for another minute until slightly softened but still crunchy.
Stir in the soy sauce, red chili sauce, tomato ketchup, and vinegar. Mix well.
Pour in the cornstarch slurry and cook until the sauce thickens and turns glossy. To make a gravy version, add 1.5 cups of water before adding the slurry.
Add the crispy fried cauliflower to the pan and toss quickly to coat each floret evenly in the hot Manchurian sauce.
Garnish with the green parts of the spring onions and serve immediately.