Mussels with Garlic and Basil (Hoi Ma Laeng Poo)

1/2 tsp. peppercorns
2 red jalape?o or serrano chilies, seeded and
coarsely chopped
2 garlic cloves, quartered
1 Tbs. chopped fresh cilantro roots or stems
3 Tbs. vegetable oil
1 1/2 Tbs. oyster sauce
1 Tbs. fish sauce, or as needed
2 lb. mussels, scrubbed and debearded
1/4 cup chicken stock
1/2 tsp. sugar
1/2 cup fresh Thai basil leaves
Fresh cilantro leaves for garnish

In a mortar, pound the peppercorns until crushed, then add the chilies, garlic and cilantro roots, and pound or grind until a rough paste forms.
Preheat a wok or fry pan over medium-high heat. When hot, add the oil and swirl to coat the pan. When the oil is hot, add the chili paste and fry until fragrant, about 30 seconds. Stir in the oyster sauce and the 1 Tbs. fish sauce. Add the mussels, discarding any that fail to close to the touch, and stir and toss with the sauce to mix. Add the stock and sugar, then cover and cook until the mussels open, about 2 minutes. Taste and adjust the seasoning with fish sauce.
Toss in the basil leaves and cook until they just begin to wilt. Transfer the mussels to a platter, discarding any that failed to open, and garnish with the cilantro leaves.