Rinse the pickled mustard greens to remove excess salt and sourness. Squeeze out the water, then cut the greens into small pieces. Cut the tomatoes into wedges. Peel the shallots and leave them whole. Sauté the white part of the green onions until fragrant, then add the mustard greens and tomatoes and stir well. Do not add seasoning since the greens are salty; just add 2 tablespoons sugar to enhance the flavor.

Add the whole shallots and crispy pork belly, stir to combine, then pour in 2 large bowls of cold water. Cover and simmer in a pressure cooker for 15 minutes. Be careful when turning off the heat—let all the steam release before opening the lid. If the lid is still tight, do not force it open as there may still be pressure. At this point, the greens should be very tender. Taste and adjust the seasoning as needed; the soup should be a little sweet and a little sour.

Slice the green onions and a few chili peppers. Serve with finely shredded lettuce.