Bring a large pot of water to a boil. Separate the mustard leaves and boil them for about 1 minute until they are just tender.

Drain the greens and spread them out on a clean kitchen towel. Once cool, roll the towel up and squeeze firmly to remove all excess moisture, then coarsely chop the leaves.

In a small pot, bring the vinegar, sugar, salt, and chile to a boil. Stir until the sugar is dissolved, then remove from heat and let the brine cool until warm.

Pack the chopped greens into a glass jar and pour the warm brine over them. Let the jar sit at room temperature for 1 day to develop flavor before refrigerating.