Heat the oil in a large tall pan. Fry the onion. Pour the curry paste, red pepper and the Pak choi in the pan. Pour the coconut milk over it. Place the mussels on top of the vegetables.
Place the mussels on top of the Pak choi and add some pepper. Cook the mussels in 6-8 min. with the lid, or until all shells are open.
Serve the mussels with some coriander and add the rice to a bowl. Serve with some lime.