While the pork is marinading, remove the mustard greens from the pack and rinse several time under running water to remove excess salt and any sand or grit in the vegetables. Cut the greens into 1/2 inch or 1.3cm pieces, place in a large bowl and rinse again (there’s a lot of rinsing, but let me tell you, nothing ruins dinner more than a mouthful of gritty, grainy, crunchy sand!). Cover the chopped greens with hot water, and leave to soak for about 1 hour, till the thicker stem pieces are soft. Drain and rinse once more, then squeeze any excess water out.