Step 1 Place the garlic and the shallot in a large skillet with the olive oil over medium heat and cook until the garlic is fragrant and the shallot has just become translucent, about 2 minutes. Swirl the pan so the garlic moves and does not burn. Add the mussels. Turn the heat to high and coat the mussels in the oil, shallot and garlic. Add a dash of salt and pepper, then the wine. Cover and simmer until the mussels have opened wide, no more than 2 minutes. Discard any unopened mussels.
Step 2 Arrange the mussels in a bowl and return the cooking liquid to the heat. Add the ong choy and the butter. Cook 1 to 2 minutes. The ong choy cooks down very quickly; you want this to remain brothy. Pour the liquid on top of the mussels and arrange the ong choy in a bundle on top. Serve while hot with the fennel salt (see related recipe) for sprinkling.