In a medium sauté pan, combine the vinegars, sugar, ¼ tsp. of the salt, and the peppercorns.
Heat over low heat, stirring, until the sugar has completely dissolved, about 3 minutes.
Remove from the heat and let cool while you prepare the mustard greens.
Meanwhile, in a large pot, bring 3 qts. water to a boil over high heat.
Add the remaining 1 tbsp. salt and the mustard green stems and boil for 1 minute.
Immediately remove the pot from the heat, add the mustard green leaves, and let sit for 1 minute. Drain in a colander and rinse with cool water.
Transfer the leaves and stems to a clean 1-qt. canning jar.
Add the chilies to the jar and pour the pickling liquid over everything. months.
Let sit at room temperature for at least 2 hours, then seal the jar and refrigerate until ready to serve.
The pickled mustard greens will keep in the refrigerator for several weeks