Add cinnamon, cloves, cumin seeds, mustard seeds, garlic, ginger, soaked red chillies, and malt vinegar to a grinder. Blend into a smooth, thick paste. The vinegar should loosen the paste without turning it watery. Set aside to allow flavours to settle.

Heat oil in a wide non-stick pan over medium heat. Add prawns in a single layer and sauté until they turn light pink. Remove immediately once just cooked. Overcooking at this stage will toughen them during final simmering.

In the same pan, add chopped onions to the remaining oil. Cook until translucent with soft edges, not browned or caramelised. This stage builds sweetness that balances the vinegar's sharpness later. Stir regularly to avoid caramelisation.

Add chopped tomato and salt. Cook until the tomato breaks down completely and turns pulpy. The oil should begin to surface lightly. This creates a stable base that prevents the vinegar paste from tasting raw.

Add the ground masala paste to the pan. Cook on medium heat, stirring continuously, until the oil separates clearly from the sides. This step is essential to soften the vinegar’s sharpness and achieve a balanced flavour.

Return prawns to the pan along with sugar. Toss gently to coat. Cook briefly so the prawns absorb the masala without releasing water. Turn off the heat once everything is evenly coated and glossy.