Wash & drain the mussels and place in a bowl with the marinade.
Peal & finely chop the garlic. Chop the white (and a bit of the green!) of the onion fairly fine. Add to the bowl and stir together.
Roughly chop the chard and put aside.
Heat the wok until its nice & hot then add the oil. When it starts to smoke, tip in the bowl contents including the marinade. Stir fry for a minute, then add the chard and continue stirring for a couple of minutes as it wilts.
Stir in the sesame oil then serve immediately with rice or noodles.