Leave whole mustard greens unwashed and wilt them in sunlight for 1 day until the leaves become limp. If sunlight is unavailable, wilt them in a ventilated area, dehydrator, or low oven at 122°F (50°C) for about 2 hours.
Bring water, sea salt, and sugar to a boil. Stir until fully dissolved, then allow the brine to cool to room temperature. Add white vinegar only after cooling if using.
Wash wilted mustard greens thoroughly, drain well, and cut into serving-size pieces. Cut scallions into sections and slice shallots thinly. Remove excess surface water before fermentation.
Clean a glass or ceramic fermentation container with hot water or boiling water. Allow the container to dry before adding vegetables.
Add mustard greens, scallions, and shallots into the jar. Pour cooled brine over the vegetables until fully submerged. Use a clean weight or plate to keep the greens below the liquid surface.
Keep the jar at room temperature for 2–5 days depending on temperature and preferred sourness. The pickle is ready when the greens turn yellow-green, develop a pleasant sour aroma, retain some crunch, and lose most of their raw bitterness.
