1. Bring the vinegar, sugar and salt to a boil in a small saucepan. Stir until the sugar and salt are dissolved. Remove from the heat and add garlic, mustard seeds, peppercorns and chili flakes if you are using them.
2. Place kale stem pieces in a large bowl and pour the vinegar mixture over it. Stir to combine spices. Make sure all the pieces are submerged in the brine. Allow to cool completely before serving.

Recipe by Robin Forte.