Place the mussels in a large pot with the Indonesian bay leaves, lemongrass, and salt. Add enough water to cover the bottom of the pot.
Cover and cook over medium-high heat until the mussels open, about 5–7 minutes. Discard any mussels that remain closed.
Drain the mussels and let them cool slightly. Remove and discard the empty half of each shell, leaving each mussel attached to one half. Arrange the mussels on a baking sheet or oven-safe baking dish.
Add the melted butter, onion, garlic, ketchup, brown sugar, lemon juice, honey, and oyster sauce to a blender or food processor. Blend until smooth.
Preheat the broiler. Spoon a generous amount of sauce over each mussel.
Broil the mussels for 3–5 minutes, or until the sauce is hot, bubbling, and lightly caramelized around the edges. Watch carefully to prevent the sauce from burning.
Serve immediately with extra lemon or lime wedges, if desired.