In a medium bowl combine the batter ingredients ( flour, cold water and salt) and whisk until smooth.
Heat frying oil in a small saucepan to 375 /190 C.
Dip cauliflower florets in the batter and coat evenly, shake off excess batter, then fry until golden brown. Transfer to a paper towel to drain excess oil.
In a small bowl mix together dark soy sauce, oyster sauce and brown sugar.
In a large skillet heat 2 tbsp of vegetable oil, saute the chopped onion until golden brown, add garlic and chili and cook until fragrant.
Add the sauce and cook for 10 seconds. Then, add the battered cauliflower florets, cashew nuts and mix to coat with the sauce.
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