Make sauce in a blender or food processor blend bell peppers, chiles, garlic, and 2 tablespoons water until smooth. Transfer to a non-aluminum saucepan. Add sugar, vinegar, oil, and salt ingredients with 2/3 cup water. Bring to a boil, then simmer for 20 minutes or until reduced by half. Leave to cool, then transfer to a jar and refrigerate to allow flavors to develop. (It can be refrigerated up to 2 weeks.)

Into a large saucepan, put clams or mussels, lemon grass, ginger, and basil sprigs. Add sufficient water to come 1-1/2 inches up sides of pan. Bring to a boil, cover, and cook over medium heat 3 to 5 minutes until shells open; discard any that remain closed. Meanwhile, into a small bowl, transfer sauce. Drain mussels or clams. Garnish with torn basil and serve with sauce.