Prepare all the ingredients: finely chop the onion and mince the garlic.
In a pan, heat oil and sauté the chopped onions and minced garlic until fragrant.
Add the black peppercorns, cut lemongrass pieces, and kaffir lime leaves. Cook for 1-2 minutes.
Stir in oyster sauce and fish sauce. Mix well to combine with the ingredients in the pan.
Add lalas (small saltwater shrimps) into the pan. Stir gently as they cook until they turn pink.
Serve immediately, garnished with lime wedges for squeezing over the dish.