First of all, pad dry the prawns with paper towel and season with simple salt and fresh ground black pepper. Break one egg white in, mix it well and set aside.
In a mixing bowl add corn starch, plain flour and baking soda. Mix well to combine all ingredients evenly. Prepare the vegetables by roughly chop the garlic cloves and cut the onion and pepper in a small cubes.
Dredge the prawns in the flour mix, press and dip gently to coat each prawns evenly. Transfer the coated prawns on the tray or pate and repeat the remaining prawns. Toss the excess flour out before frying.
In a small sauce pan, add chicken stock, ketchup, vinegar, sugar, salt, tomato puree and corn starch. Stir well with medium heat until the sugar dissolved and the sauce has thicken slightly. Remove from heat and keep it warm.
Heat the vegetable oil to high heat about 350°F, deep fry the prawns until crispy light brown. Remove from oil and transfer to paper towel.
In a large wok/pan, heat 1 tablespoon of oil with medium-high heat, stir fry the chopped garlic for 10 seconds. Then follow with the onion and peppers continue fry for another 1 minute.
Place the fried prawns back in the wok, pour the sweet and sour sauce in. Toss well to combine the prawns, sauce and vegetables evenly for 1-2 minutes.
Remove from heat and sprinkle the chopped spring onions. Transfer to serving plate and serve immediately!