Cut off a thin slice of the root end of the mustard greens and discard. Separate the leaves and wash well with water. Dirt tends to stick on the lower stems. After washing the mustard greens, cut stems and leaves into 1-inch segments.

Place the mustard greens in a large bowl, sprinkle with salt (1 tablespoon), and massage gently. Allow it to sit for 30 minutes.

In the meantime, make the brine. In medium stock pot, bring water to a boil. Turn off heat then add sugar, and remaining salt (2 tablespoons). Mix well. Set aside to cool until warm.

Rinse the greens well under cold water to remove all the salt and drain dry. Evenly divide them between two jars. Layer in the sliced shallots and whole chili peppers as you fill the jars. Press down firmly to remove any air pockets. Pour in the warm brine until it completely submerges the vegetables. There will be extra brining liquid. Save it for another pickle recipe or simply discard. Cover the opening of the jar with saran wrap then screw on lid.

Store at room temperature for about 3-4 days. Once the mustard greens have reached your desired sourness level, store them in the refrigerator. It should turn a dull green/yellow color over time with brine remaining clear. They can be kept for up to two months in the fridge. Enjoy with Thịt Kho Tàu, a braised pork belly and egg dish or any other protein dish.