Place the prawns and pork belly into two separate mixing bowls. In each bowl, add 1 tablespoon of fish sauce, 1/2 tablespoon sugar, 1/2 teaspoon freshly ground black pepper, 1 tablespoon garlic, and 1 tablespoon lemongrass.
Toss to combine the ingredients in each bowl well and marinate for 15 minutes.
In a hot fry pan, add the oil and fry the red shallots and the remaining 2 tablespoons of lemongrass until fragrant. Now add the pork belly and cook on high heat for 2 minutes, or until brown. Add the prawns and stir fry until the prawns change color, then add the spring onions and toss for another minute.
Transfer the mixture to a plate and garnish with cilantro and serve with steamed jasmine rice.